ter·ti·ar·y 
(tûrshē-ĕr′ē)
(tûrshē-ĕr′ē)adj.
1. Third in place, order, degree, or rank.
2. Of, relating to, or designating the short flight feathers nearest the body on the rear edge of a bird's wing.
3. Chemistry
a. Of, relating to, or having a carbon atom that is attached to three other carbon atoms in a molecule.
b. Of or relating to the replacement of three of several atoms or groups in a compound, such as an amine in which three valences of the functional group are taken by carbon atoms.
c. Of or relating to salts of acids containing three replaceable hydrogen atoms.
4. Tertiary Of, relating to, or being the period of geologic time from about 66 to about 2.6 million years ago, the older of the two periods of the Cenozoic Era. The Tertiary Period is characterized by the appearance of modern flora, insects, fish, amphibians, and reptiles, and by the dominance of mammals as vertebrate life forms on land. See Table at geologic time.
n. pl. ter·ti·ar·ies
1. A tertiary feather.
2. Tertiary The Tertiary Period.
3. Roman Catholic Church A member of a religious Third Order.
[Latin tertiārius, from tertius, third; see trei- in the Appendix of Indo-European roots.]