An enum, short for "enumerated type," is a special data type that consists of a set of named values called elements, members, enumeral, or enumerators of the type. Enums are useful because they allow programmers to define a set of named constants that represent a fixed number of possible values for a variable.

In Java, for example, an enum type is defined using the enum keyword and can include methods and other fields. The names of an enum types fields are usually in uppercase letters because they are constants. Enums can be accessed using the dot syntax, and they are often used in switch statements to check for corresponding values.

In C, the enum keyword is combined with a typedef to name the type. In Rust, enums are used to describe tagged unions and are much more flexible than in other languages. In Dart, enums are supported in a basic form and have a syntax similar to other languages.

Overall, enums are useful for defining a set of named constants that represent a fixed number of possible values for a variable. They are supported in many programming languages and can be used to make code more readable and maintainable.