pentad(n = noun.quantity) 5, cinque, fin, five, fivesome, little phoebe, phoebe, quint, quintet, quintuplet, v - the cardinal number that is the sum of four and one;
is a kind of digit, figure

1.  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. [1913 Webster]

2.  Any grouping of five things. [PJC]

   Having the valence of a pentad. [1913 Webster]

pentad, n.
1 the number five.
2 a group of five.

Etymology:
Gk pentas -ados f. pente five