In mathematics, an inflection point is a point on a curve at which the curve changes from being concave upward to being concave downward, or vice versa. A concave upward curve can be thought of as one that would hold water, while a concave downward curve is one that would not. An important qualification is that the curve must have a unique tangent line at the point of inflection. This means that the curve must change smoothly from concave upward to concave downward, not abruptly. As a practical example of an inflection point consider an "s-curve" on the highway. Precisely at the inflection point the driver changes from steering left to steering right, or vice versa as the case may be.