In mathematics, a point where the concavity of a curve changes from concave upward to concave downward is called an inflection point. In more mathematical terms, it is also a point where the second derivative of the curve changes from positive to negative or vice versa, as the example linked in wolfram-alpha shows.
When the marginal returns are increasing, the profit curve is concave up (has a positive second derivative) and when marginal returns are diminishing, the profit curve is concave down (has a negative second derivative). Thus the transition meets the definition of an inflection point.