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The Collaborative International Dictionary
Penta-

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.

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