And let's say that this is x equals b, just like that. And what we want to do is concern ourselves with the area under the graph, under the graph of y is equal to f of x, and above the x-axis, and between these two bounds, between x equals a and x equals b. So this area right over here. And you can already get an appreciation. We're not used to finding areas where one of the boundaries, or as we'll see in the future, many of the boundaries could actually be curves. But that's one of the powers of the definite integral, and one of the powers of integral calculus. And so the notation for this area right over here would be the definite integral.