The Collaborative International Dictionary of English v.0.48:

Pyro- \Pyro-\, Pyr- \Pyr-\ . [Gr. ?, ?, fire.]
   Combining forms designating fire or heat; specifically
   (Chem.), used to imply an actual or theoretical derivative by
   the action of heat; as in pyrophosphoric, pyrosulphuric,
   pyrotartaric, pyrotungstic, etc.
   [1913 Webster]