السلام عليكم ورحمة الله
al-Salāmu ʿalaykum wa raḥmatu llāh
“Peace be upon you and the mercy of God”: said whenever a Muslim meets a Muslim and on entering a house or room.
What it means
This is the Muslim greeting in its fuller form, and the short form is al-salāmu ʿalaykum. The reply is wa ʿalaykumu l-salām wa raḥmatu llāh. The Quran commands, “When you are greeted with a greeting, return it with a better one or the same” (4:86), and it tells people entering houses to greet one another with a greeting from God, blessed and good (24:61). A hadith in Sahih Muslim, reported by Abu Hurayra, tells believers to spread the greeting among themselves, and it names this as a means of mutual love.
Often misunderstood as
The words are sometimes heard as a password for insiders, whereas the greeting is a prayer for peace and mercy offered to every person addressed.
Term and Arabic as listed in Ismail al Faruqi, Toward Islamic English (International Institute of Islamic Thought, 1995), p. 59, listed as AL SALĀMU ‘ALAYKUM WA RAḤMAH ALLAH.. The definition is restated here.