OAM (Object Attribute Memory)
The small block of memory where a video chip keeps its sprite attributes — position, tile number and colour — one row per sprite.
The NES PPU (1983) set aside 256 bytes for OAM: 64 sprites of four bytes each — Y position, pattern number, attributes (palette, flip, priority) and X position. The CPU rarely wrote it byte by byte; instead a whole page was copied in via DMA ($4014) at once, since OAM could only be touched safely during vertical blank. The same idea returned on the Game Boy, SNES and Game Boy Advance with more sprites and more attribute bits. Sprite 0 — the first entry — held a special place: its collision with the background was the only mid-frame timing signal NES code had.