Pattern table
The actual 8×8 pixel tiles the NES draws from — two bitplanes per tile give four colours, and the nametable points here.
Where the nametable says which tile goes where, the pattern table stores what the tile actually looks like. Each tile is 8×8 pixels encoded as two bitplanes — 16 bytes — giving four possible values per pixel. The PPU holds 256 background tiles and 256 sprite tiles at once.
The table usually sat in its own ROM chip on the cartridge (CHR-ROM), costing nothing from the console's scarce RAM. Games with extra mapper chips swapped whole pattern tables mid-frame to show more than the 256 unique tiles at a time.