A "constant" is a value that does not change. In mathematics, constants are fixed numbers, such as π (pi), which is approximately 3.14, or e, the base of natural logarithms, approximately 2.71. Constants can also appear in equations, where they represent unchanging quantities. In programming, a constant is a variable whose value cannot be altered after it has been assigned. This helps prevent accidental changes to important values, such as MAX_VALUE in a program, which might represent the highest limit for a calculation. Using constants improves code readability and maintainability.