In C programming, an enum (short for "enumeration") is a user-defined data type that consists of a set of named constants. enum values are typically used to represent a set of related constants that have a fixed value and are not likely to change.

To define an enum in C, you use the enum keyword followed by a set of constant names enclosed in braces. For example, the following code defines an enum named Month that contains the names of the twelve months:

enum Month {
    JANUARY,
    FEBRUARY,
    MARCH,
    APRIL,
    MAY,
    JUNE,
    JULY,
    AUGUST,
    SEPTEMBER,
    OCTOBER,
    NOVEMBER,
    DECEMBER
};

Each constant in the enum is assigned a value that starts at 0 for the first constant and increments by 1 for each subsequent constant. You can also explicitly assign values to enum constants, like this:

enum Day {
    MONDAY = 1,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY
};

In this example, the MONDAY constant is assigned the value 1, and the subsequent constants are assigned values that increment by 1.

To declare a variable of an enum type, you simply use the enum name followed by the variable name. For example, the following code declares a variable named currentMonth of type Month and initializes it to MARCH:

enum Month {
    JANUARY,
    FEBRUARY,
    MARCH,
    APRIL,
    MAY,
    JUNE,
    JULY,
    AUGUST,
    SEPTEMBER,
    OCTOBER,
    NOVEMBER,
    DECEMBER
};

enum Month currentMonth = MARCH;

enum values can be compared using the == and != operators, but you cannot perform arithmetic operations on enum values.

enum values are typically used in switch statements and to improve code readability by using descriptive names instead of hard-coded numeric constants.