Constant

Constants in programming are variables in which their value does not change during the runtime of the program.

Compile-time constants

Some programming languages evaluate constants at compile time. This means that their value is worked out by the compiler and is then hardcoded into the binary.

For these compile-time constants, they can't use any functions or data that is only available at runtime. For example, compile-time constants can't take in user input or call any other non-constant function.