decad

plural decads

1. archaicArchaic form of decade (“period of ten years”).

   Since Averill was a decad and a half / His elder, and their parents underground

   1864, Alfred Tennyson, “Aylmer’s Field”, in Enoch Arden, &c., London: Edward Moxon & Co., […], →OCLC, page 55

2. archaicArchaic form of decade (“group of ten”).

   Plato […] extends number as far as to the decad.

   1806, Aristotle, translated by Thomas Taylor, The Physics, or Physical Ausculation of Aristotle, page 167

3. entertainmentA group of ten notes from which the consonant triads may be constructed