Tileset
Also said as Tilemap, Autotile, Tile bitmask
A set of small square images that fit together edge to edge, used to build big levels from a small amount of art. Autotiling picks the right edge or corner piece for you by checking each tile's neighbours.
One tileset can make hundreds of levels, in very little memory. Without autotiling, a level designer places every corner by hand, and every map edit means fixing the edges around it.
Say it
“Use 16 pixel tiles with a four-neighbour autotile. That is 16 pieces of ground, chosen by checking up, right, down and left, and three variants of the plain middle so large areas do not look stamped.”
Where you’ve seen it
RPG Maker's editor calls them autotiles: you paint with one grass or water tile, and it picks the shores and corners for you.
Not to be confused with sprite
Tiles stay on a grid and make up the level. Sprites are free to move anywhere on top of it.