Left to choice; not compulsory or automatic.(adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "optional" in a sentence
"The word optional was neither uttered nor implied."
"Its most recent issue, however, features what it calls "optional inflation hedges" - for investors who want additional protection beyond that offered by its actively managed "Upgraded" portfolios."
"While i agree that the idea of making senior year optional is a terrible one … it is not as offensive as one of the other mentioned ideas"