A const (constant) is a type of variable in programming that cannot be changed after it has been initialized. This means that once a value is assigned to a const, it remains the same throughout the program. Using const helps prevent accidental changes to important values, making the code more reliable and easier to understand. In many programming languages, such as C++, Java, and JavaScript, declaring a variable as const is done using specific syntax. For example, in C++, you would write `const int x = 10;`. This indicates that the value of `x` will always be 10, ensuring consistency in calculations and logic throughout the code.