Avada Kedavra, also known as the killing curse, is one of the three Unforgivable Curses. Performing these curses are the most heavily punished crimes by wizarding law. When the curse is cast it produces a flash of blinding green light and a rushing sound. The victims are left unmarked but dead. There is no known countercurse to Avada Kedavra and it can not be blocked. No person has ever survived from this curse without dying except, of course, Harry Potter. 
/əˈvɑːdə kəˈdɑːvrə/ ə-vah-də kə-dah-vrə
Causes instant, painless death to whomever the curse hits. There is no countercurse or method of blocking this spell; however, if someone sacrifices their life for someone else, the person who was saved will not encounter any adverse effects of any curses by the specific attacker (e.g. when Lily Potter sacrificed her life for Harry Potter at Voldemort's hands, Harry became immune to curses cast by Voldemort). One of the three Unforgivable Curses.
Only two people in the history of the magical world are known to have survived the killing curse – Harry Potter and Voldemort who was only saved by his horcrux. Harry was hit twice directly. Phoenixes can also survive a killing curse. They burst into flame, as they would do in old age and are reborn from the ashes. This occurred in Harry Potter and the Order of the Phoenix.
First said (not by name) at the beginning of the first book when Harry arrives at the Dursleys' home. Nearly cast on Harry by Lucius Malfoy near the end of the second film. Seen first in Goblet of Fire against Muggle Frank Bryce, and in every book following.
 Suggested etymology
During an audience interview at the Edinburgh Book Festival (15 April 2004) Rowling said: "Does anyone know where avada kedavra came from? It is an ancient spell in Aramaic, and it is the original of abracadabra, which means 'let the thing be destroyed.' Originally, it was used to cure illness and the 'thing' was the illness, but I decided to make it the 'thing' as in the person standing in front of me. I take a lot of liberties with things like that. I twist them round and make them mine." Rowling's use of this name may have been influenced by Latin cadaver = "corpse".