IWAD

An internal WAD, or IWAD, is the main WAD file for a Doom-based game. It contains all the sprites, levels, music, other graphics, multimedia data, and various important lump files that a particular game uses. The main examples are Doom, Doom 2 and both parts of Final Doom, but in SRB2's case, it would be srb2.srb, which lacks player sprites, maps and music (these are stored externally in PWADs).