elective

Concise Oxford English Dictionary © 2008 Oxford University Press:
elective/ɪˈlektɪv/

▶adjective

• 1 relating to or appointed by election.

• 2 (of a course of study) chosen by the student rather than compulsory.

  ■ (of surgical or medical treatment) chosen by the patient rather than urgently necessary.

▶noun chiefly N. Amer. an optional course of study.

– derivatives
electively adverb.