Any item considered to be of third order.

A tertiary colour.

Something from the Tertiary Period (the former term for the geologic period from 65 million to 2.58 million years ago).

Of third rank or order; subsequent.

Possessing some quality in the third degree; especially having been subjected to the substitution of three atoms or radicals.

“a tertiary alcohol, amine, or salt”

Of quills: growing on the innermost joint of a bird's wing; tertial.