The period derivative is the rate at which the period of some cycle is changing as time passes, a period such as an orbital period or an object's rotation period. It is typically indicated by a P with a dot over it, often pronounced p-dot:
. P (meaning dP/dt)
Examples with astrophysical interest include period derivatives regarding the rotation of pulsars, which gradually slow down, a rate of use when using pulsars for timing (e.g., for pulsar timing arrays), and period derivatives of the orbits around compact objects, such as that of the Hulse-Taylor Binary which demonstrated the effects of gravitational waves.