A Definition File (def file for short) is a text file with the .def extension that defines various properties of the game. There are four types of Declarations you will find in def files.
EntityDefinition - Blocks of the form
entityDef defName { ... }
ModelDefinition - Blocks of the form
model modelName { ... }
ModelExportBlock - Blocks of the form
export exportName { ... }
MapDefinition - Blocks of the form
mapDef mapName { ... }
Examples
player.def Quake 4's Player Definition File. This file defines almost all information pertaining to the player in Quake 4. Max health, starting weapons, what models are valid in multiplayer, etc.