Subinterval: A subinterval is a smaller interval within a larger interval. It represents one slice or segment in which we divide our overall interval.
Area under curve: The area under a curve represents the total sum of all the areas covered by rectangles in an approximation method like Riemann sums.
Function evaluation: This refers to finding out what value a function takes for a specific input or x-value. In midpoint Riemann sums, function evaluations are done at midpoints to determine heights of rectangles.