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Undefined variable
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<p>An undefined variable in the <a href="/facts/Source_code/o9J6Jv23">source code</a> of a <a href="/facts/Computer_program/e38jH9N9">computer program</a> is a <a href="/facts/Variable_(programming)/VoCKtwml">variable</a> that is accessed in the code but has not been <a href="/facts/Declaration_(computer_science)/CbH6UjHA">declared</a> by that code. </p><p>In some <a href="/facts/Programming_languages/cWTYbgWa">programming languages</a>, an implicit declaration is provided the first time such a variable is encountered at <a href="/facts/Compile_time/yCdI3oqn">compile time</a>. In other languages such a usage is considered to be sufficiently serious that a diagnostic being issued and the compilation fails. </p><p>Some language definitions initially used the implicit declaration behavior and as they matured provided an option to disable it (e.g. <a href="/facts/Perl/fx31kjlT">Perl</a>'s "use warnings" or <a href="/facts/Visual_Basic/7QMXM6fP">Visual Basic</a>'s "Option Explicit"). </p>