I have to mention that T might not be used at all, for example with the JAXA's launch of the Epsilon rocket, no designator was used since there was only a single system clock to synchronize with (something that didn't go quite according to plans with the first launch attempt of this new launcher and the timestamp discrepancy between the onboard timekeeper and the mission control of a few milliseconds caused the launch sequence to be halted and postponed to a later date). More about its purpose can be read in this thread: What is the purpose of having a countdown during a rocket launch?

So in short, T it's just a convention, a single letter abbreviated designator for the system clock (mostly quoted to stand for Time) that a lot of events are synchronised with throughout the duration of the launch.

There are other time designators used, for example L can stand for time before the launch day, E for the time to some event, encounter or ETA (Estimated Time of Arrival), and so on. T-0 does not necessarily mean the exact time in this sequence when the liftoff happens, and can also stand for the time of main engine ignition, or anything similar, though usually close to the actual liftoff and rocket's clearing of the launch tower. Countdown technically ends with T-0 ( T minus zero) and the count then continues in the positive range ( T plus 1, 2.), but we usually refer to all of it as the countdown for simplicity sake, even after continuing to count upwards with other sequences taking place post launch. T is just a designator for the main sequence countdown time that serves as the synchronisation device for the majority of systems and procedures that have to take place before, exactly on and even after the countdown sequence reaches T-0. 17.8k1313 gold badges6262 silver badges142142 bronze badges

What does t minus 3 days mean