What does T stand for in a rocket launch?

Answer 1

T means the Time of launch -- when the launch is planned to happen. T minus is the number of seconds before the rocket is to launch. T minus four seconds and counting means it is four seconds in the sequence of event before launch will occur and progressing toward that launch. T minus four seconds and holding means it is four seconds before launch in the sequence of events that precede launch and the progress toward launch has stopped. The hold/delay can be most any length of time, but once counting begins again it will still be four seconds until launch -- that is, they pick up where they left off and continue toward launch.

There are both planned holds (to give everyone time to evaluate things) and unplanned holds when something is wrong. After launch it becomes T plus so many seconds. Actions during launch happen so many seconds after launch. For example, on a Space Shuttle launch, the Solid Rocket Boosters will separate from the Shuttle at T plus so many seconds.