What is "first-class citizen"

n. 1 A member of a class of individuals that receive fair treatment. 2 (context programming languages English) (synonym of first-class object English)

In programming language design, a first-class citizen (also type, object, entity, or value) in a given programming language is an entity which supports all the operations generally available to other entities. These operations typically include being passed as an argument, returned from a function, and assigned to a variable.