Tile flipping
Storing a flip flag per cell in a tilemap so a single tile can be drawn mirrored horizontally, vertically or diagonally — halving the tileset you need to author.
A symmetric wall needs no separate tile for its left edge and its right; the right one is just the left mirrored. By keeping a few bits beside each tile index — flip-X, flip-Y, flip-diagonal — the same image can be reused in four to eight orientations. The NES, SNES and Mega Drive all carried these flags in the hardware's attribute bytes.
The Tiled editor keeps them in the top three bits of each 32-bit GID, which is why a tile index in practice only has 29 bits. The trick saves both VRAM and art effort, and is often combined with metatiles and autotiling.