In short
Pulling away from a stationary position on an incline without rolling back.
A hill start is the manoeuvre for pulling away on an incline without the car rolling backward. In a manual: clutch bite + gas + release the parking brake. In an automatic (most US test cars): brake-hold or hill-assist keeps the car still long enough to switch to the accelerator.
Significant roll-back is a critical error in most US states.
See also