The keyword const in programming languages like C++ and JavaScript is used to declare variables whose values cannot be changed after they are assigned. This means that once a variable is defined as const, it cannot be reassigned to a different value, helping to prevent accidental modifications and making the code easier to understand and maintain. Using const can improve code safety and clarity, as it signals to other developers that the variable is intended to remain constant throughout its scope. This practice is especially useful in large codebases, where keeping track of variable changes can become complex.