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