In contrast to a variable, a constant represents a fixed, non-changeable value. In programming, the value of the constant should not change through the program flow.
A constant has a unique identifier, a data type and the fixed value from the value system. In the programming the identifier and the data type are agreed by declaration. With it the definitions of dimensions, data types and variables are meant. Such a declaration protects the constant against changes.
Typical examples for constants are tax rates like the value added tax, the number "e", "pi" or the prefix for capital "K".