1. Noun. A planet in Mass Effect where you must sacrifice one of two squadmates, Kaidan Alenko or Ashley Williams. Other notable events include an encounter with main villains Saren and Sovereign, the detonation of a nuclear bomb, and the possible death of Urdnot Wrex, another squadmate.
2. Verb. The act of sacrificing someone on Virmire.

to tell someone to calm down and relax.