—adj. 1. of the third order, rank, stage, formation, etc.; third.
2. Chem. a. noting or containing a carbon atom united to three other carbon atoms.
b. formed by replacement of three atoms or groups.
3. (cap.) Geol.noting or pertaining to the period forming the earlier part of the Cenozoic Era, occurring from 65 million to 2 million years ago, characterized by the development and proliferation of mammals. See table under geologic time. 4. Ornith.tertial.
5. Eccles.noting or pertaining to a branch, or third order, of certain religious orders that consists of lay members living in community (regular tertiaries) or living in the world (secular tertiaries).
—n. 1. (cap.) Geol.the Tertiary Period or System.
2. Ornith.a tertial feather.
3. (often cap.)Eccles.a member of a tertiary branch of a religious order.
4. See tertiary color.