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. adjective
    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
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    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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