Etymology: (1645–55; < Gk pentad- (s. of pentás) group of five. See pent -, -ad1)
Definition of 'pentad'
Princeton's WordNet
1. (noun)five, 5, V, cinque, quint, quintet, fivesome, quintuplet, pentad, fin, Phoebe, Little Phoebe the cardinalnumber that is the sum of four and one
2. (noun)pentad any element, atom, or radical, having a valence of five, or which can be combined with, substituted for, or compared with, five atoms of hydrogen or other monad; as, nitrogen is a pentad in the ammonium compounds
Definitions of pentad 
