optional

Misollar

Pain is inevitable. Suffering is optional.
To the US, international laws are optional.
The recipe mentions nutmeg, but it's optional.
In Lojban, verbal tense and aspect are optional.
Growing old is mandatory. Growing up is optional.
The car is equipped with numerous optional accessories.

ot noun

Something that is not compulsory, especially part of an academic course.
(sometimes capitalized) In various programming languages, a kind of variable that is assigned a specific data type but may or may not hold an actual value.

sifat adjective

Not compulsory; left to personal choice; elective. On that beach clothing is entirely optional.

Manbalar: Wiktionary (Free Dictionary API orqali), Datamuse, Tatoeba (CC BY 2.0 FR), MyMemory.