In some games the existence of some entities in the gameworld may be determined by the field-of-view of the player. Whenever an entity leaves the players field of view it is removed automatically from the world. We call this cropping in analogy to the process in which parts of a photograph are eliminated.
Cropping is typically seen in shooter games where the players bullets/laser beams/etc are removed from the gameworld once they reach the edge of the screen. It is thus impossible, when this sort of cropping occurs, for the player to hit enemy entities that are off-screen.
Grand Theft Auto: San Andreas - In this game, pedestrians in the city only exist within the field of view. This can be easily noticed if the camera angle changes suddenly, and pedestrians that were walking on the street are gone, or have been joined by a number of other pedestrians.