Let be a differentiable function, and let be its derivative. The derivative of (if it has one) is written and is called the "second derivative of ". Similarly, the derivative of the second derivative, if it exists, is written and is called the "third derivative of ". Continuing this process, one can define, if it exists, the th derivative as the derivative of the th derivative. These repeated derivatives are called "higher-order derivatives". The th derivative is also called the derivative of order .