Texture atlas

A texture atlas is an arrangement of game sprites on a single image file, used by the game to store and access associated sprites more efficiently than with individual files. Minecraft uses both procedurally-generated and predefined texture atlases for different purposes.

Procedurally-generated texture atlases
Block and item textures are combined into the same atlas. It is not known where particle, painting and other individual textures are combined.

bars.png
bars.png is used for storing boss bar textures and their overlays. As of Java Edition 1.16.2, this file is still in use.

icons.png
icons.png is used for storing the textures of certain UI elements.

As of Java Edition 1.16.2, this file is still used in the game.

terrain.png
terrain.png was used for storing the textures of blocks.

items.png
items.png was used for storing the textures of items

particles.png
particles.png was/is used for storing the textures of most particles. Two other atlases, explosion.png and sweep.png, were used for respectively storing explosion and sweeping attack particles.

kz.png
kz.png was/is used for storing the textures of paintings.