As-salamu alaykum , also written salamun alaykum and typically rendered in English as salam alaykum, is a greeting in Arabic that means 'Peace be upon you'. The salām (سَلَام, meaning 'peace') has become a religious salutation for Muslims worldwide when greeting each other, though its use as a greeting predates Islam, and is also common among Arabic speakers of other religions (such as Arab Christians and Mizrahi Jews).
In colloquial speech, often only salām, 'peace', is used to greet a person. This shorter greeting, salām (سَلَام), has come to be used as the general salutation in other languages as well.
The typical response to the greeting is wa-ʿalaykumu s-salām (وَعَلَيْكُمُ ٱلسَّلَامُ [wa.ʕa.laj.ku.mu‿s.sa.laːm] ⓘ, 'and peace be upon you'). In the Quranic period one repeated as-salamu alaykum, but the inverted response is attested in Arabic not long after its appearance in Hebrew. The phrase may also be expanded to as-salāmu ʿalaykum wa-raḥmatu -llāhi wa-barakātuhᵘ̄ (ٱلسَّلَامُ عَلَيْكُمْ وَرَحْمَةُ ٱللَّٰهِ وَبَرَكَاتُهُ [as.sa.laː.mu ʕa.laj.kum wa.raħ.ma.tu‿ɫ.ɫaː.hi wa.ba.ra.kaː.tu.hu], 'Peace be upon you, as well as the mercy of God and His blessings').
The use of salām as an Arabic greeting dates at least to Laqit bin Yamar al-Ayadi (6th century), and cognates in older Semitic languages—Aramaic šlāmā ʿalḵōn (ܫܠܵܡܵܐ ܥܲܠܟ݂ܘܿܢ) and Hebrew shalom aleichem (שָׁלוֹם עֲלֵיכֶם shālôm ʻalêḵem)—can be traced back to the Old Testament period.