This comes from a countdown convention used in by both the American military and NASA.

Generally, it is used when counting down to a major event that will happen at a specific time.

Mathematically, T is time, minus whatever amount of time is left until the event happens.

If the News Years ball is dropping in 10 minutes, one could say "The ball is dropping in T minus 10 minutes and counting!"

Therefore, "T minus zero" means that there is no time left.

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One side has the word, one side has the definition. Microwave and dishwasher safe. Lotsa space for your liquids.

Buy the mug### The Urban Dictionary Mug

One side has the word, one side has the definition. Microwave and dishwasher safe. Lotsa space for your liquids.

Buy the mug