Elective /(?)/

E·lect·ive

Elective

a.
  1. Exerting the power of choice; selecting; as, an elective act.
  2. Pertaining to, or consisting in, choice, or right of choosing; electoral.
    The independent use of their elective franchise.
  3. Bestowed or passing by election; as, an elective office.
    Kings of Rome were at first elective; . . . for such are the conditions of an elective kingdom.
  4. Dependent on choice; that can be refused; as, an elective college course. Opposite of required or mandatory.

Phrases & Compounds
Elective affinity
a tendency to unite with certain things; chemism.

Elective

n.
  1. In an American college, an optional study or course of study; a course that is not required. [Colloq.]