A constant in computer programming is a type of variable whose value is defined once and cannot be changed throughout the program's execution. Constants are used to store values that remain the same, such as mathematical constants like pi (π), configuration values, or fixed values needed in the program. Using constants improves code readability and reduces the possibility of errors, as their values are set only once and protected from unintended modifications. Different programming languages have different ways of defining constants, but they are a fundamental concept across various programming paradigms.