pentad noun. ['pɛntad] M17.
[Greek pentad, -as from pente five: see -AD¹.]
1. The number five in the Pythagorean system. M17.

2. Chemistry. Orig., a pentavalent element or group. Now, a pentameric unit within a polymer. M19.

3. A group of five; spec. (a) a period of five years (cf. DECADE 3); (b) Meteorology a period of five days. L19.

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