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 are used in equations and formulas to represent unchanging quantities. In programming, a constant is a variable whose value cannot be altered after it has been assigned. For example, in the programming language Python, you might define a constant for the number of days in a week as `DAYS_IN_WEEK = 7`. This ensures that the value remains the same throughout the program, preventing accidental changes.