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<h2 id="metadata">Metadata</h2> <p>In metadata, an identifier is a language-independent label, sign or token that uniquely identifies an object within an <a href="/facts/Identification_scheme/YwUqdNRT">identification scheme</a>. The suffix "identifier" is also used as a <a href="/facts/Representation_term/clVjSMY5">representation term</a> when naming a <a href="/facts/Data_element/jtyoAIAd">data element</a>. </p><p>ID codes may inherently carry <a href="/facts/Metadata/qt35z79C">metadata</a> along with them. For example, when you know that the food package in front of you has the identifier "2011-09-25T15:42Z-MFR5-P02-243-45", you not only have that data, you also have the metadata that tells you that it was packaged on September 25, 2011, at 3:42pm UTC, manufactured by Licensed Vendor Number 5, at the Peoria, IL, USA plant, in Building 2, and was the 243rd package off the line in that shift, and was inspected by Inspector Number 45. </p><p>Arbitrary identifiers might lack metadata. For example, if a food package just says 100054678214, its ID may not tell anything except identity—no date, manufacturer name, production sequence rank, or inspector number. In some cases, arbitrary identifiers such as sequential serial numbers leak information (i.e. the <a href="/facts/German_tank_problem/xMSy97YB">German tank problem</a>). Opaque identifiers—identifiers designed to avoid leaking even that small amount of information—include "really <a href="/facts/Opaque_pointer/NNc0Nm1J">opaque pointers</a>" and <a href="/facts/Universally_unique_identifier/SUtkYJPe">Version 4 UUIDs</a>. </p> <h2 id="in-computer-science">In computer science</h2> <p>In <a href="/facts/Computer_science/lN3pEyPz">computer science</a>, identifiers (IDs) are <a href="/facts/Lexical_(semiotics)/EV2rqeBx">lexical</a> <a href="/facts/Token_(parser)/T1JYWpIf">tokens</a> that name <a href="/facts/Entity_class/U8A0mwYe">entities</a>. Identifiers are used extensively in virtually all information processing systems. Identifying entities makes it possible to refer to them, which is essential for any kind of symbolic processing. </p> <h3>In computer languages</h3> <p class="note">Main article: <a href="/facts/Identifier_(computer_languages)/mBLrnPkj">Identifier (computer languages)</a></p> <p>In <a href="/facts/Computer_language/n3L56qdP">computer languages</a>, identifiers are <a href="/facts/Token_(parser)/T1JYWpIf">tokens</a> (also called <a href="/facts/Symbol/GS8mTn0h">symbols</a>) which name language entities. Some of the kinds of entities an identifier might denote include <a href="/facts/Variable_(programming)/VoCKtwml">variables</a>, <a href="/facts/Data_type/fcx5hgVL">types</a>, <a href="/facts/Label_(programming_language)/DT6KU8R5">labels</a>, <a href="/facts/Subroutine/9vFJrg0A">subroutines</a>, and <a href="/facts/Modular_programming/sNoqKvRD">packages</a>. </p> <h2 id="ambiguity">Ambiguity</h2> <h3>Identifiers (IDs) versus Unique identifiers (UIDs)</h3> <p class="note">Main article: <a href="/facts/Unique_identifier/NMcgc7qX">Unique identifier</a></p> <p>A resource may carry multiple identifiers. Typical examples are: </p> <ul><li>One person with multiple names, nicknames, and forms of address (titles, salutations) <ul><li><i>For example:</i> One specific person may be identified by all of the following identifiers: Jane Smith; Jane Elizabeth Meredith Smith; Jane E. M. Smith; Jane E. Smith; Janie Smith; Janie; Little Janie (as opposed to her mother or sister or cousin, Big Janie); Aunt Jane; Auntie Janie; Mom; Grandmom; Nana; Kelly's mother; Billy's grandmother; Ms. Smith; Dr. Smith; Jane E. Smith, PhD; and Fuzzy (her jocular nickname at work).</li></ul></li> <li>One document with multiple versions<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a></li> <li>One substance with multiple names (for example, <a href="/facts/Chemical_Abstracts_Service/CVLrS5IJ">CAS</a> index names versus <a href="/facts/IUPAC/2AV6myCK">IUPAC</a> names;<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> <a href="/facts/International_Nonproprietary_Name/wYL655x6">INN</a> generic drug names versus <a href="/facts/United_States_Adopted_Name/5AFEKGfQ">USAN</a> generic drug names versus brand names)</li></ul> <p>The inverse is also possible, where multiple resources are represented with the same identifier (discussed below). </p> <h3>Implicit context and namespace conflicts</h3> <p class="note">See also: <a href="/facts/Naming_collision/cLswdvPn">Naming collision</a></p> <p>Many <a href="/facts/Code/CSntvnEo">codes</a> and <a href="/facts/Nomenclature/YhTgjjyY">nomenclatural</a> systems originate within a small <a href="/facts/Namespace/9efXwMtV">namespace</a>. Over the years, some of them bleed into larger namespaces (as people interact in ways they formerly had not, e.g., cross-border trade, scientific collaboration, military alliance, and general cultural interconnection or assimilation). When such dissemination happens, the limitations of the original naming convention, which had formerly been latent and moot, become painfully apparent, often necessitating <a href="/facts/Retronym/yBLuNepw">retronymy</a>, <a href="/facts/Synonym/a3BgMcdu">synonymity</a>, translation/<a href="/facts/Transcoding/5GHL1F4B">transcoding</a>, and so on. Such limitations generally accompany the shift away from the original context to the broader one. Typically the system shows implicit context (context was formerly assumed, and narrow), lack of capacity (e.g., low number of possible IDs, reflecting the outmoded narrow context), lack of <a href="/facts/Extensibility/AEPWNwxp">extensibility</a> (no features defined and reserved against future needs), and lack of specificity and disambiguating capability (related to the context shift, where longstanding uniqueness encounters novel nonuniqueness). Within computer science, this problem is called <a href="/facts/Naming_collision/cLswdvPn">naming collision</a>. The story of the origination and expansion of the <a href="/facts/CODEN/YmfJcNjb">CODEN</a> system provides a good case example in a recent-decades, technical-nomenclature context. The capitalization variations seen with <a href="/facts/Proper_noun/bMbxQSn7">specific designators</a> reveals an instance of this problem occurring in <a href="/facts/Natural_language/B4gLS7Kd">natural languages</a>, where the proper noun/common noun distinction (and its complications) must be dealt with. A universe in which every object had a UID would not need any namespaces, which is to say that it would constitute one gigantic namespace; but human minds could never keep track of, or semantically interrelate, so many UIDs. </p> <h2 id="identifiers-in-various-disciplines">Identifiers in various disciplines</h2> <p class="note">Main page: Category:Identifiers</p> <table><tbody><tr><th>Identifier</th><th>Scope</th></tr><tr><td><a href="/facts/Atomic_number/bgAr581S">atomic number</a>, corresponding one-to-one with <a href="/facts/List_of_chemical_element_name_etymologies/SC6vo2rK">element name</a></td><td>international (via <a href="/facts/International_scientific_vocabulary/y6VJSmaJ">ISV</a>)</td></tr><tr><td><a href="/facts/Australian_Business_Number/sALMkq6h">Australian Business Number</a></td><td>Australian</td></tr><tr><td><a href="/facts/Commercial_and_Government_Entity/ubuEmXo6">CAGE code</a></td><td>U.S. and <a href="/facts/NATO/dCeTddMY">NATO</a></td></tr><tr><td><a href="/facts/CAS_registry_number/zjXjCGPx">CAS registry number</a></td><td>originated in U.S.; today international (via <a href="/facts/International_scientific_vocabulary/y6VJSmaJ">ISV</a>)</td></tr><tr><td><a href="/facts/CODEN/YmfJcNjb">CODEN</a></td><td>originated in U.S.; today international</td></tr><tr><td><a href="/facts/Digital_object_identifier/muM9Etpq">Digital object identifier</a> (DOI, doi)</td><td><a href="/facts/Handle_System/rdebSxmC">Handle System</a> <a href="/facts/Namespace/9efXwMtV">Namespace</a>, international scope</td></tr><tr><td><a href="/facts/Deutsches_Institut_f%25C3%25BCr_Normung/BGemsWax">DIN</a> standard number</td><td>originated in Germany; today international</td></tr><tr><td><a href="/facts/E_number/n3VSY29I">E number</a></td><td>originated in E.U.; may be seen internationally</td></tr><tr><td><a href="/facts/Enzyme_Commission_number/Y5GNLRKX">EC number</a></td><td></td></tr><tr><td><a href="/facts/Employer_Identification_Number/SR5cNsdA">Employer Identification Number</a> (EIN)</td><td>U.S.</td></tr><tr><td>Electronic Identifier Serial Publicaction (EISP)</td><td>international</td></tr><tr><td><a href="/facts/Global_Trade_Item_Number/NKewx6Bj">Global Trade Item Number</a></td><td>international</td></tr><tr><td><a href="/facts/Group_identifier/23lndCN4">Group identifier</a></td><td>many scopes, e.g., specific computer systems</td></tr><tr><td><a href="/facts/International_Chemical_Identifier/6yR96JYR">International Chemical Identifier</a></td><td>international</td></tr><tr><td><a href="/facts/International_Standard_Book_Number/15AdSPa9">International Standard Book Number</a> (ISBN)</td><td>ISBN is part of the <a href="/facts/International_Article_Number_(EAN)/1BqwBFU2">EAN</a> <a href="/facts/Namespace/9efXwMtV">Namespace</a>; international scope</td></tr><tr><td>International eBook Identifier Number (IEIN)</td><td>international</td></tr><tr><td><a href="/facts/International_Standard_Serial_Number/DPAflDvU">International Standard Serial Number</a> (ISSN)</td><td>international</td></tr><tr><td><a href="/facts/International_Organization_for_Standardization/IacYR7Yg">ISO</a> standard number, e.g., <a href="/facts/ISO_8601/4N8kykG9">ISO 8601</a></td><td>international</td></tr><tr><td><a href="/facts/Library_of_Congress_Control_Number/5hb5Cw2V">Library of Congress Control Number</a></td><td>U.S., with some international bibliographic usefulness</td></tr><tr><td><a href="/facts/Personal_identification_number_(Denmark)/q67Nslvf">Personal identification number (Denmark)</a></td><td>Denmark</td></tr><tr><td><a href="/facts/Pharmaceutical_code/1CooEeps">Pharmaceutical code</a></td><td>Many different systems</td></tr><tr><td>Product batch number</td><td></td></tr><tr><td><a href="/facts/Serial_Item_and_Contribution_Identifier/wMqQMJ39">Serial Item and Contribution Identifier</a></td><td>U.S., with some international bibliographic usefulness</td></tr><tr><td><a href="/facts/Serial_number/2XUENAvJ">Serial number</a></td><td>many scopes, e.g., company-specific, government-specific</td></tr><tr><td>Service batch number</td><td></td></tr><tr><td><a href="/facts/Social_Security_Number/MPmrfRg2">Social Security Number</a></td><td>U.S.</td></tr><tr><td><a href="/facts/Tax_file_number/YPHnRr8y">Tax file number</a></td><td>Australian</td></tr><tr><td>Unique Article Identifier (UAI)</td><td>international</td></tr></tbody></table> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Barcode/Mkh4cCTp">Barcode</a></li> <li><a href="/facts/Binomial_nomenclature/jgBqAhPv">Binomial nomenclature</a></li> <li><a href="/facts/British_Approved_Name/pm1t52SQ">British Approved Name</a></li> <li><a href="/facts/Data_descriptor/mptmlgTB">Data descriptor</a></li> <li><a href="/facts/Data_element/jtyoAIAd">Data element</a></li> <li><a href="/facts/Descriptor_(disambiguation)/Lnc04W5z">Descriptor</a></li> <li><a href="/facts/Diagnosis_code/n6CMd9oD">Diagnosis code</a></li> <li><a href="/facts/Document_management_system/cgmjwyKV">Document management system</a></li> <li><a href="/facts/File_descriptor/Iw74UKqG">File descriptor</a></li> <li><a href="/facts/Food_labeling_regulations/q7tQ3zLk">Food labeling regulations</a></li> <li><a href="/facts/Gene_nomenclature/IYLpdHbP">Gene nomenclature</a></li> <li><a href="/facts/Handle_(computing)/gKL1X2sX">Handle (computing)</a></li> <li><a href="/facts/Identification_(disambiguation)/7BMoHyNu">Identification</a></li> <li><a href="/facts/Identity_(object-oriented_programming)/M8Pua97X">Identity (object-oriented programming)</a></li> <li><a href="/facts/Identity_document/YovHMC8K">Identity document</a></li> <li><a href="/facts/Index_term/fRqKUggI">Index term</a></li> <li><a href="/facts/Marketing_part_number/768WlxIE">Marketing part number</a></li> <li><a href="/facts/Metadata/qt35z79C">Metadata</a></li> <li><a href="/facts/Name_binding/HbBx9mpM">Name binding</a></li> <li><a href="/facts/Namespace/9efXwMtV">Namespace</a></li> <li><a href="/facts/Naming_convention_(programming)/4IVCWpSU">Naming convention (programming)</a></li> <li><a href="/facts/National_identification_number/0frd2aya">National identification number</a></li> <li><a href="/facts/Nomenclature/YhTgjjyY">Nomenclature</a> – contains various standardized naming systems</li> <li><a href="/facts/Nomenclature_code/UIalq4GC">Nomenclature code</a> <ul><li><a href="/facts/Chemical_nomenclature/scv3SqmM">Chemical nomenclature</a></li> <li><a href="/facts/International_Code_of_Nomenclature_for_algae%2c_fungi%2c_and_plants/EbaUF9P5">International Code of Nomenclature for algae, fungi, and plants</a></li> <li><a href="/facts/International_Code_of_Nomenclature_of_Bacteria/oGfYGIxN">International Code of Nomenclature of Bacteria</a></li> <li><a href="/facts/International_Code_of_Nomenclature_for_Cultivated_Plants/PHhY50WN">International Code of Nomenclature for Cultivated Plants</a></li> <li><a href="/facts/International_Code_of_Zoological_Nomenclature/Pxg8gsqf">International Code of Zoological Nomenclature</a></li></ul></li> <li><a href="/facts/Function_overloading/wL27p4rD">Overloading</a></li> <li><a href="/facts/Part_number/nhgPWMC2">Part number</a></li> <li><a href="/facts/Personally_identifiable_information/384PbBg5">Personally identifiable information</a></li> <li><a href="/facts/Product_code_(disambiguation)/gKF1v67x">Product code</a></li> <li><a href="/facts/Reference_(computer_science)/1vm2Bph1">Reference (computer science)</a></li> <li><a href="/facts/Referent/QfURtq1V">Referent</a></li> <li><a href="/facts/Representation_term/clVjSMY5">Representation term</a></li> <li><a href="/facts/Systematized_Nomenclature_of_Medicine/Px5Svo4e">Systematized Nomenclature of Medicine</a></li> <li><a href="/facts/Uniform_resource_identifier/UjyqxTpA">Uniform resource identifier</a> (URI)</li> <li><a href="/facts/Unique_identifier/NMcgc7qX">Unique identifier</a></li> <li><a href="/facts/Unique_key/NKGBxUHR">Unique key</a></li></ul> <h2 id="external-links">External links</h2> <ul><li> The dictionary definition of identifier at Wiktionary</li> <li> Media related to Identifiers at Wikimedia Commons</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>University of Glasgow. "Procedure for Applying Identifiers to Documents". Archived from the original on 5 June 2011. Retrieved 28 April 2009. <a href="/wiki/University_of_Glasgow" target="_blank">/wiki/University_of_Glasgow</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>University of Pennsylvania. "Information on Chemical Nomenclature". Archived from the original on 4 January 2009. Retrieved 28 April 2009. <a href="/wiki/University_of_Pennsylvania" target="_blank">/wiki/University_of_Pennsylvania</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> </ol>