Texture atlas
Also said as Atlas, Sprite atlas, Texture packing
One big image with many smaller images packed into it. Switching images is slow for a graphics card, so if everything lives in a single image the game can draw it all in one go.
Every switch can force a new draw call, which is a separate instruction to the graphics card. Atlases turn hundreds of draw calls into a few, which is often the difference between a smooth game and a stuttering one.
Say it
“Pack all the UI icons into one 2048 pixel atlas with 2 pixels of padding, so the inventory screen is a single draw call and not 60.”
Where you’ve seen it
Stardew Valley keeps its items together on one sheet, springobjects.png, and item mods have long patched that single file.
Not to be confused with sprite sheet
A sprite sheet keeps one thing's frames in a neat grid for animation. An atlas packs anything together, in any arrangement, for speed.