optional

Etymology

option + -al

adj

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

noun

1. Something that is not compulsory, especially part of an academic course.
2. (programming, 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.
   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, page 147