Speeding Ticket: What is the legal definition of "pacing"?

I am wondering if there is a legal definition of “pacing” when used as a method of assessing a car’s speed for a traffic ticket outlined anywhere (online or in print)? Particulary asking for the state of California, but a US Federal guideline could be sufficient as well.

Anything such as outlined proceedures, distance requirements, time duration minimums, etc?