A program used to edit Doom levels.
The format of Doom levels has been disclosed not long after the game was released, so that players can design their custom levels. Not long after that, many level editors sprang up into existence. Because Doom levels are stored in WAD files, they are also called WAD editors.
Most editors shows the level in a 2-D top-down view, and the user can add/delete walls, texture walls, floors and ceilings, set the height of floors and ceilings, add monsters, create doors/switches/other kinds of active things, set the light level of areas, and so on. When the user finishes designing a level, he save it in a PWAD (patch WAD) file, generate some rendering information using a so-called node builder, then he can play with the new level. Such editors are also used to look into a level, including original Doom levels, in order to know how to get out a level, or to gain some insight in level design.
Most editors runs on DOS, such as DEU. A level editor that runs on linux/X11 is called yadex.