The Collaborative International Dictionary
Penta-
Penta- \Pen"ta-\ [Gr. ?, a later combining form of ? five. See Five.]
A combining form denoting five; as, pentacapsular; pentagon.
(Chem.) Denoting the degree of five, either as regards quality, property, or composition; as, pentasulphide; pentoxide, etc. Also used adjectively.
Douglas Harper's Etymology Dictionary
penta-
word-forming element meaning "five, containing five," from Greek penta- (before a vowel pent-), comb. form of pente "five," related to Aeolian pempte (see five), with -a- by analogy of hepta-, ennea-, deka-.
Wiktionary
penta-
pre. five