optional

adjectives

possible but not necessary; left to personal choice
optional
antonym
obligatory
is similar to
not compulsory
"elective surgery"; "an elective course of study"
elective
is similar to
as a favor; not compelled by legal right
"ex gratia payments made to nonstriking workers"
ex gratia
is similar to
not compulsory
"facultative courses in the sciences"
facultative
is similar to
not required by rule or law
nonmandatory, nonobligatory