Create a Sprite Atlas

To create a SpriteA 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 Atlas:

1. Go to Assets > Create > 2D > Sprite Atlas.

Unity creates the Sprite AtlasGraphics:A utility that packs several sprite textures tightly together within a single texture known as an atlas. More info. 2D: A 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 in the Assets folder, with the file extension .spriteatlas.

SpriteAtlas