What's the "een" in Halloween?
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-.
penta- or pent-
Containing five atoms, molecules, or groups: pentose.