Derivative
The instantaneous rate of change of a function at a point.
The derivative f'(a) measures how fast f(x) is changing at x = a, defined as the limit of the average rate of change over an interval shrinking to zero. Geometrically it is the slope of the tangent line to the graph at that point.
Formula
f'(a) = lim h→0 (f(a + h) − f(a)) / h