(usually writing? In mathematics, especially in calculus and vector analysis, it means partial derivative. Partial derivative describes the rate of change of multivariate function in the direction of one variable, while keeping other variables unchanged. In vector analysis and physics, partial derivatives are often used to describe the changes of scalar fields or vector fields in space. For example, in thermodynamics, the temperature gradient is a vector field, and its component is the partial derivative of temperature with respect to various spatial coordinates.