Halo did not define it, because Perfect Dark came oh so many years before Halo, and was a greater game, had a great plot, and didn't milk itself with millions of games.viranimus said:What defined it? People. Primarily with their reaction to them. Otherwise, it is just a disc.
Oh you want a specified game?
NO! you get 3.
Birthed it? Doom. Enough said.
Defined it? Halo (specifically 2 when console multiplayer became widespread and increased the popularity of shooters across the board)
Redefined it? Bioshock. Proper shooter with hybrid elements of RPGs with in depth and cohesive story and char building it has advanced the genera as a whole, because some people who wont play a shooter, will play bioshock willingly.
EDIT: you could argue that Kill.Switch redefined it, with the advent of the cover system, or gears of war with perhaps the fluidity and need to utilize the cover system. I personally wouldnt, but it is a valid argument.
Perfect Dark defined FPS gaming, since it improved on everything that Goldeneye was, even though Goldeneye was still a major game.
I don't know if you should say Bioshock "redefined" it, fps games are still games where you go around shooting things, except Bioshock just had a better story and way to play than most others.