What defines a constant in programming?

A constant in programming is defined as a value that never changes throughout the execution of a program. Once a constant is assigned a value, that value cannot be altered or reassigned, ensuring that it remains the same under all circumstances during the program's runtime. This feature of constants is particularly useful when you want to create values that are meant to remain fixed, such as mathematical constants (like Pi), configuration settings, or any other values that should not be modified accidentally during the course of a program's operation.

Using constants helps increase code reliability and readability, as programmers can easily identify values that are intended to remain unchanged, thus preventing unintended errors. Additionally, this characteristic can improve performance because the compiler can optimize the code knowing that certain values will not change.