Penta- \Pen"ta-\ [Gr. ?, a later combining form of ? five. See Five.]

1. A combining form denoting five; as, pentacapsular; pentagon.

2. (Chem.) Denoting the degree of five, either as regards quality, property, or composition; as, pentasulphide; pentoxide, etc. Also used adjectively.

Source: Webster's Revised Unabridged Dictionary (1913)