An
Arabic phrase used by
Muslims everywhere as a greeting. It means 'Peace be upon you.'
'As-Salaamu' means "The Peace" and 'Alaikum' means "On you." The proper response is "Wa Alaikum As-Salaam," which translates to "And upon you be peace.' People use this phrase when meeting someone, and when leaving. You can say this to one person or a group. Saying this to someone is said to give you 10 blessings.
The full greeting is "Assalamu Alaikum wa rahmatullahi wa barakatuhu." Peace be upon you and the mercy of God and His blessings. Saying this full phrase is said to have a reward of 30 blessings. It's a good idea to reply to the person with the same phrase, i.e. if the greeter uses the full phrase, you should reply in kind, the same for the short.
The word Salaam (Peace) sounds very similar to the Hebrew word Shalom, which also means Peace.
Even though the phrase is in Arabic, you can use it anywhere. If you go to a mosque in Pakistan or the US, you can say it and be understood. People wish random strangers peace all the time, since Muhammad(pbuh)'s last speech said that every Muslim is a brother to one another. When a person says it to you or replies, then you can be sure they are a Muslim.
On the internet, people usually shorten it to Salaam in a chat. You can say "Salaam" and have most people in a room say "Wa Salaam" back to you.