1. third, 3rd, tertiary -- (coming next after the second and just before the fourth in position)
tertiary can be used as a noun
1. Tertiary, Tertiary period -- (from 63 million to 2 million years ago)
1.
He might dissect, anatomize, and give names but, not to speak of a final cause, causes in their secondary and tertiary grades were utterly unknown to him. - from Frankenstein by Mary Wollstonecraft (Godwin) Shelley