Examples from the Web for pentad
Nitrogen in the triad condition in the amines is far less poisonous than in the pentad condition.Poisons: Their Effects and Detection|Alexander Wynter Blyth
Five, or the pentad, is everything; it stops the power of poisons, and is dreaded by evil spirits.Three Thousand Years of Mental Healing|George Barton Cutten
There is something to be said for this view, but on the whole the balance of the evidence is in favor of nitrogen being a pentad.
But this oddness belongs to other numbers also (pentad, heptad, &c.).Aristotle|George Grote
In 1820, he added to the last a pentad, so that the whole list amounted to sixty-five.Man and His Migrations|R. G. (Robert Gordon) Latham
British Dictionary definitions for pentad
Word Origin for pentad
Word Origin and History for pentad
1650s, from Greek pentas (genitive pentados) "group of five" (see five). Meaning "period of five years" is from 1880; meaning "period of five days" is from 1906, originally in meteorology.