What does "pyrosulphuric" mean?

Pyrosulphuric: Pertaining to, or designating, an acid called also disulphuric acid) obtained by distillation of certain sulphates, as a colorless, thick, oily liquid, H2S2O7 resembling sulphuric acid. It is used in the solution of indigo, in the manufacture of alizarin, and in dehydration.