tertiary

Concise Oxford English Dictionary © 2008 Oxford University Press:
tertiary /ˈtəːʃ(ə)ri/

▶adjective

• 1 third in order or level.

  ■ chiefly Brit. relating to or denoting education at a level beyond that provided by schools.

• 2 relating to or denoting medical treatment provided at a specialist institution.

• 3 (Tertiary) Geology relating to or denoting the first period of the Cenozoic era (between the Cretaceous and Quaternary periods, about 65 to 1.64 million years ago).

• 4 Chemistry (of an organic compound) having its functional group on a carbon atom bonded to three other carbon atoms.

  ■ Chemistry derived from ammonia by replacement of three hydrogen atoms by organic groups.

▶noun a lay associate of certain Christian monastic organizations.

– origin C16: from L. tertiarius ‘of the third part or rank’.