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