1. The problem asks: What is a constant? 2. A constant is a value that does not change. In mathematics, it is a fixed number. 3. For example, in the expression $y = 3x + 5$, the number $5$ is a constant because it stays the same no matter what $x$ is. 4. Constants are important because they provide a reference or fixed value in equations and functions. 5. Unlike variables, constants do not vary or depend on other values. Final answer: A constant is a fixed value that does not change.