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.2004, Paula Begoun, The Complete Beauty Bible: The Ultimate Guide to Smart Beauty, Rodale Inc., →ISBN, page 419:
Something that is not compulsory, especially part of an academic course.
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Computing, Engineering, Mathematics, Natural sciences, Physical sciences, Programming, SciencesCapitalized, Sometimes 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.2015, Richard M Reese, Learning Java Functional Programming, Packt Publishing Ltd, →ISBN, page 147: