Sprite Atlas

A 2D project uses spritesA 2D graphic objects. If you are used to working in 3D, Sprites are essentially just standard textures but there are special techniques for combining and managing sprite textures for efficiency and convenience during development. More info
See in Glossary and other graphics to create the visuals of its scenesA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
See in Glossary. This means a single project can contain many texture files. Unity typically issues a draw call for each texture in the scene; however, in a project with many textures, multiple draw calls become resource-intensive and can negatively impact the performance of your project.

A Sprite AtlasA texture that is composed of several smaller textures. Also referred to as a texture atlas, image sprite, sprite sheet or packed texture. More info
See in Glossary is an asset that consolidates several textures into a single combined texture. Unity can call this single texture to issue a single draw call instead of multiple draw calls to access the packed textures all at once at a smaller performance overhead. You can use the Sprite Atlas API to control loading the Sprite Atlases at your project’s runtime.