What is a scatter diagram in which the dots form a kind of upside down U shape?

In math this is called a parabola (with a negative coefficient on the squared term) but it suggests that the relationship is non-linear, specifically quadratic (the data is best explained by a squared term). In practical terms, this means that the relationship is one in which medium levels of the x variable result in the highest levels of the y variable. Stress and performance are a good example of this. Your performance is highest when you have medium amounts of stress. If low, you are unmotivated, if high, you choke. A second way of thinking of this is as an interaction (with itself) (the effect of stress on performance DEPENDS on the amount of stress you have).