1. [from LISP terminology for 'true'] Yes. Used in reply to a question (particularly one asked using the '-P' convention). In LISP, the constant T means 'true', among other things. Some hackers use 'T' and 'NIL' instead of 'Yes' and 'No' almost reflexively. This sometimes causes misunderstandings. When a waiter or flight attendant asks whether a hacker wants coffee, he may well respond 'T', meaning that he wants coffee; but of course he will be brought a cup of tea instead. As it happens, most hackers (particularly those who frequent Chinese restaurants) like tea at least as well as coffee -- so it is not that big a problem.
3. [techspeak] In transaction-processing circles, an abbreviation for the noun 'transaction'.
4. [Purdue] Alternate spelling of tee.