optional

adj. left to one's choice, not compulsory, voluntary, discretionary

Adjective
1. possible but not necessary; left to personal choice
(antonym) obligatory
(similar) elective

adj. optionally, in optional way; via choice

n. option, choice

(n.)
See Elective, n.
  

(a.)
Involving an option; depending on the exercise of an option; left to one's discretion or choice; not compulsory; as, optional studies; it is optional with you to go or stay.