Once upon a time, long long ago*, Quake Live was known as Quake Zero. Quake Zero was like Quake I, Quake II, Quake III Arena and Quake 4, but 1, 2, 3 arenas and 4 less respectively. After realising the negative impact a consecutive sequence of version numbers has on the nature of the universe it was decided that the Quake series would now use words for version identification. Live, Living, Dead, Undead, Resurrected, etc.
* Some time in 2007