MWI

The MWI is essentially a key to an MWD file; it contains information about every file in the archive. It is generally located in a game's executable. It also contains file entries for files inside of WADs.

Etymology
It is believed that MWI stands for MediEvil WAD Index or Millennium WAD Index. The acronym WAD was coined during the development of Doom and stands for Where's All the Data?