WadC is a programming language designed for constructing Doom levels. It was written by Wouter "Aardappel" van Oortmerssen.
The system takes the form of a two pane window. In the left pane, WadC code can be written. A preview of the resulting level is displayed in the right pane.
WadC has advantages over traditional level editors in that it is easy to form repeating structures: a function can be written to create a pillar, for example, and invoked to create many pillars.
Perhaps the best known WadC maps are by none other than Aardappel himself, such as his submission to Doomworld's 10 Sectors competition, or Crucified Dreams MAP12. The source to both are included as in the WadC zip file as