An easy-to-understand definition of the Internet term T1. This is a data transfer system that transfers digital signals at 1.544 megabits per second (quite a bit faster than a 56K modem, which maxes out at around 0.056 Mbps). General Information We have chosen to discuss T4, T4A and T5 payroll slips because they are the most common ones that we see during personal tax season. The information from these slips are inputted into tax software to prepare your T1 personal tax return. Definition of T1 in Slang/Internet Slang. What does T1 stand for? Printer friendly. Menu Search. New search features Acronym Blog Free tools "AcronymFinder.com. Aug 30, 2016 · What is the T1 General Tax Form? Also called T1, General Income Tax Form or Income Tax and Benefit Return, the T1 tax form is what individual Canadians use to complete their personal income tax return. It has five sections: Identification: Where you give information like your name, your address, your SIN and your marital status.