What is the term for a reaction time of exactly 0.000 seconds?

A reaction time of exactly 0.000 seconds is called a perfect light.

Reaction time measures the interval between the starting signal and the vehicle leaving the staging beams. A displayed 0.000 means the driver’s launch occurred exactly at the timing system’s reference point, with neither an early red-light violation nor a measurable delay shown by the timing display.

The phrase “perfect light” does not mean the driver completed the run fastest. Reaction time and elapsed time are separate measurements: reaction time covers the launch, while elapsed time measures the car’s trip down the track. A driver can record a perfect light and still lose if the car is slower or has a mechanical problem.

Drag-racing timing systems also distinguish a perfect light from a red light. Leaving before the permitted start produces a negative reaction time and triggers disqualification. “Full tree” refers instead to the sequential amber-light starting procedure used in many sportsman classes; it is not the name for a 0.000 reaction time.