The Collaborative International Dictionary
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.
Douglas Harper's Etymology Dictionary
before vowels pyr-, word-forming element form meaning "fire," from Greek pyro-, combining form of pyr (genitive pyros) "fire, funeral fire," also symbolic of terrible things, rages, "rarely as an image of warmth and comfort" [Liddell & Scott]; see fire (n.). Pyriphlegethon, literally "fire-blazing," was one of the rivers of Hell.
alt. 1 fire, heat. 2 fever. 3 (context chemistry English) orthoacid. pre. 1 fire, heat. 2 fever. 3 (context chemistry English) orthoacid.