optional

adj

1. (not-comparable)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.”

noun

1. Something that is not compulsory, especially part of an academic course.
2. (capitalized, 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.”