 More precisely, a straight line is said to be a tangent of a curve at a point on the curve if the line passes through the point on the curve and has slope , where "f" is the derivative of "f".A similar definition applies to space curves and curves in "n"-dimensional Euclidean space.As it passes through the point where the tangent line and the curve meet, called the point of tangency, the tangent line is "going in the same direction" as the curve, and is thus the best straight-line approximation to the curve at that point.