There are two definitions by which this can be used. Typically in NANOGrav we talk about how the pulse profile changes slightly as a function of frequency. That is, the pulse shape looks different from one frequency to the next and thus we have to make sure to account for this when creating TOAs, and we now parameterize them with FD parameters. However, we are also concerned that there is no evolution of the profile in time since we want our pulses to be very stable. This stability has been tested.