Sprite sheet
A single image packing many sprites or animation frames, addressed by UV coordinates to cut texture binds and draw calls.
Instead of one file per sprite, every frame sits in a grid inside one image, and the game points at a sub-rectangle with UV coordinates when it draws. An animation becomes a row of frames you step through; a character set becomes a sheet you index into. The format goes back to the sprite hardware of the 1970s and 80s, where sprites were small fixed-size cels — the "sheet" is the software-era generalisation.
The payoff is both practical and performance-related: the texture is already bound, so the engine avoids state changes between frames and can merge the drawing (see sprite batching). The free-size variant, where arbitrary textures are packed tightly, is a texture atlas.