In C programming language, a constant is a named memory location that holds a single value throughout the execution of the program. Constants are similar to variables, but their values cannot be altered or changed during the execution of the program. In C, constants can be of any of the basic data types like an integer constant, a floating constant, a character constant, or a string literal. The type of constant can be an integer constant, a floating pointer constant, a string constant, or a character constant. There are two simple ways in C to define constants: using the #define preprocessor or using the const keyword. The #define preprocessor is used to define a constant with the following form: #define identifier value. On the other hand, the const keyword is used to define a constant variable that cannot be modified once it is declared in the program.