The Collaborative International Dictionary of English v.0.48:

Propidene \Pro"pi*dene\, n. [Propyl + ethylidene.] (Chem.)
   The unsymmetrical hypothetical hydrocarbon radical,
   CH3.CH2.CH, analogous to ethylidene, and regarded as the
   type of certain derivatives of propane; -- called also
   propylidene.
   [1913 Webster]