Tileset
The palette of individual tile images a tilemap indexes into — the pictures, as opposed to the grid of indices.
A tilemap is just an array of integers; the tileset is the lookup table they point into. The tiles usually share a uniform size — 8×8 pixels on the NES, 16×16 or 32×32 common in modern 2D games — and are packed into a single image so the GPU can draw the whole map in few draw calls.
Modern tools make the tileset more than pictures. The tileset resources in Tiled and Godot attach per-tile metadata: collision shapes, animation frames, and the neighbour masks that autotiling reads. Separating tileset from tilemap is the whole point — the same grid can change its entire mood by pointing at a different tileset.