Sprite batching
Merging many sprites that share a texture into a single draw call via one vertex buffer, instead of issuing a call per sprite.
Every draw call carries CPU overhead, and a 2D scene can hold thousands of sprites. The batcher gathers the ones that share a texture and render state, builds one vertex buffer with all their corners, and submits the lot in a single call. XNA's SpriteBatch (2006) made the pattern canonical, and every 2D engine now ships an equivalent.
The whole trick lives or dies on not breaking the batch. A texture swap, a shader swap or a blend-mode change forces a new batch — which is exactly why sprite sheets and texture atlases matter: keep everything on one texture and a whole screen can collapse to one call. Related but distinct is GPU instancing, which draws many copies of the same geometry.