In programming, a constant is a value that cannot be changed during the execution of a program. For example, if you define a constant for the number of days in a week, it will always be 7. This helps prevent accidental changes to important values, making your code more reliable and easier to understand. Constants are often used for settings or configurations that should remain the same throughout the program. Constants are typically defined using specific keywords, depending on the programming language. In Java, you might use the keyword `final`, while in Python, you can simply use uppercase letters to indicate a constant. By using constants, developers can create cleaner and more maintainable code, reducing the risk of errors.