In computer programming, a constant is a value that cannot be altered by the program during normal execution, i.e., the value is constant. When associated with an identifier, a constant is said to be "named," although the terms "constant" and "named constant" are often used interchangeably. This is contrasted with a variable, which is an identifier with a value that can be changed during normal execution, i.e., the value is variable. Constants are useful for both programmers and compilers: for programmers they are a form of self-documenting code and allow reasoning about correctness; while for compilers they allow compile-time and run-time checks that constancy assumptions are not violated, and allow or simplify some compiler optimizations.

const float PI = 3.1415927;  // maximal single float precision

const unsigned int MTU = 1500;  // Ethernet v2, RFC 894

const unsigned int COLUMNS = 80;