The Collaborative International Dictionary of English v.0.48:

Penta- \Pen"ta-\, pref. [Gr. penta-, a later combining form of
   pe`nte five. See Five.]
   1. A combining form denoting five; as, pentacapsular;
      pentagon.
      [1913 Webster]

   2. (Chem.) Denoting the degree of five, either as regards
      quality, property, or composition; as, pentasulphide,
      containing five sulfur atoms; pentoxide, containing five
      oxygen atoms, etc. Also used adjectively.
      [1913 Webster]