Partial derivative
Also: partial derivatives, ∂f/∂x
The derivative of f with respect to one variable, treating every other variable as a constant, written ∂f/∂x. The gradient is the vector of all of them.
∂f/∂x = lim h→0 [f(x + h, y) − f(x, y)] / h
Example
For f(x, y) = x²y at (3, 2): ∂f/∂x = 2xy = 12 and ∂f/∂y = x² = 9, so f is more sensitive to x than to y at that point.