optional

Not compulsory; left to personal choice; elective.

On that beach clothing is entirely optional.

Do you need to wear eyeliner? As with any makeup step, eyeliner is completely optional. From an artistic perspective, if you are wearing eyeshadow, I almost always recommend wearing eyeliner, unless your eyelids and eyelashes are obscured by the eyebrow area. Eyeliner is a basic part of an eye-makeup design because it shapes and defines the eyes and makes the eyelashes look thicker.

Something that is not compulsory, especially part of an academic course.

In various programming languages, a kind of variable that is assigned a specific data type but may or may not hold an actual value.

In functional languages, there is a data type called the option type, also called the maybe type. Its purpose is to encapsulate a value, which is returned from a function. […] In Java 8, the Optional class has been added to serve as an option type.