optional

If you can choose whether or not to do something, it is optional. Extra credit is always optional; death and taxes are not.

When you buy a new car, you can decide which options or choices, you would like. Things like a DVD player mounted in the ceiling are optional, you don't need them for the car to work, and it's up to you if you want to pay more for them. The opposite of optional is mandatory. If you're in high school, going to bed at 8:00 is optional, for your little sister in elementary school, it's mandatory.

Definitions of optional
1

adj possible but not necessary; left to personal choice

Synonyms:
elective
not compulsory
ex gratia
as a favor; not compelled by legal right
facultative
not compulsory
nonmandatory, nonobligatory
not required by rule or law
Antonyms:
obligatory
morally or legally constraining or binding
bounden
morally obligatory
compulsory, mandatory, required
required by rule
de rigueur
required by etiquette or usage or fashion
imposed
set forth authoritatively as obligatory
incumbent on
morally binding or necessary
indispensable
unavoidable
prerequisite
required as a prior condition or course of study
necessary
absolutely essential
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