What is the definition of tailgating?

Two seconds.

The average person takes two seconds to see a problem, thing about it and commit to a reaction, then react. It doesn't matter what vehicle you're driving - if you are closer than two seconds to the vehicle in front you do not have time to stop in an emergency. Back off immediately and build space so you do have time to react to problems.

Cars should stay a minimum of 4 seconds back from the vehicle in front of them while trucks need to keep a minimum of 7 seconds to maximize their stop time. The first two seconds to react to a problem and the rest to move or stop the vehicle. Maximize your space so you can control it if there is an emergency.