Climaxes are the final missions of a Grand Theft Auto Game. They consist of the player battling a large force of rival gang and/or mafia members until the end where the player kills the game's antagonist. In most cases, the weather of the climax depends on the player's timing, but in a few cases the weather is predetirmined by the game. 2 examples are Grand Theft Auto: San Andreas's End of the Line or Liberty City Stories's The Sicilian Gambit.
Grand Theft Auto IV, due to its 2 endings, has 2 climaxes depending on the player's actions. in the "Deal" ending, Antagonist Dimtiri Rascalov is seen killing fellow antagonist Jimmy Pegorino. In the "Revenge" ending, Dimitri is already dead a few missions prior and Niko kills Pegorino himself.
Climaxes usually span across severaly parts of the in-game world during some sort of chase, but will finally end in a confrontation between the protagonist and antagonist.
Similar to a Climax is a mission that involves a climactic battle of some sort in the game before the actual climax where the player usually kills a secondary antagonist. Some examples would be San Andreas's Pier 69 where CJ kills secondary antagonist Ryder and minor antagonist T-Bone Mendez or Grand Theft Auto: Vice City Stories's Light My Pyre where Victor Vance kills antagonist Armando Mendez in his own home.