Partial derivatives (seen written as the symbol δ) are derivatives of a function containing multiple variables that have all but the variable of interest held fixed during the differentiation. Thus, when a function *f* (*x*, *y*, …) depends on more than one variable, the partial derivative can be used to specify the derivative with respect to one or more variables. There are other terms, too: Partial derivatives that involve more than one variable are called *mixed partial derivatives.* And a differential equation expressing one or more quantities in terms of partial derivatives is called, logically, a *partial differential equation.* These equations are well known in physics and engineering, and most are notoriously difficult to solve.