left to one's choice; not required or mandatory: Formal dress is optional.
leaving something to choice.
Origin of optional
Synonyms for optional
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
possible but not compulsory; left to personal choice
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Online Etymology Dictionary, © 2010 Douglas Harper