elective

Short definition from Wiktionary

Adjective

1. 1
   Of, or pertaining to voting or elections; involving a choice between options.
2. 2
   Open to choice; freely chosen; (also, usually) unnecessary; minor.
3. 3
   Scheduled and nonemergent (regardless of whether necessary or unnecessary and whether minor or serious).

Noun

1. 1
   Something that is an option or may be freely chosen, especially a course of study.