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]