What is the use of "pyro" in the naming of organic compound? Recently I asked a question on the naming of polyhydroxy phenols. One of them was pyrogallol (1,2,3-Trihydroxybenzene) which is  synthesized from gallic acid  obtained from oak galls. (BEWARE- not a compound of gallium). 


But why "pyro"? Pyro is usen in naming inorganic acids like pyrosulfuric acid ($\ce{H2S2O7}$) or pyrophosphoric acid ($\ce{H4P2O7}$). It is also observed that the carboxylic acid derivative is gallic acid not pyrogallic acid and the hydroxy derivative is pyrogallol not gallol. Why is it so?
So, as @long and @permeakra stated, the name pyrogallol came from the fact that it is obtained by heating gallic acid. The IUPAC goldbook clearly states that:- 


  "pyro" is used as a prefix designating compounds formed by heating a compound,
  usually with the elimination of water, carbon dioxide, or other simple
  molecule, e.g. pyroglutamic acid from glutamic acid.


So as per rule, it is pyrogallol and not gallol.