The Collaborative International Dictionary of English v.0.48:

Isoprene \I"so*prene\, n. [Etymol. uncertain.] (Chem.)
   An oily, volatile unsaturated hydrocarbon (C5H8), obtained
   by the distillation of caoutchouc or gutta-percha, and used
   to make synthetic rubber by polymerization. In organic
   chemistry, it is viewed conceptually as the building block of
   the terpene series of hydrocarbons.
   [1913 Webster +PJC]