Being of the third formation, order, or rank; third; as,
a tertiary use of a word.

Possessing some quality in the third degree; having been
subjected to the substitution of three atoms or radicals; as, a
tertiary alcohol, amine, or salt. Cf. Primary, and Secondary.

Later than, or subsequent to, the Secondary.

Growing on the innermost joint of a bird’s wing; tertial;
– said of quills.

A member of the Third Order in any monastic system; as,
the Franciscan tertiaries; the Dominican tertiaries; the Carmelite
tertiaries. See Third Order, under Third.

The Tertiary era, period, or formation.

One of the quill feathers which are borne upon the basal
joint of the wing of a bird. See Illust. of Bird.