T1 or T-1 is used for various purposes as a designation, abbreviation or identifier:

• In telecommunications see DS1 and T-carrier.
• In Canada, "T1" the form number for the personal income tax return.
• In topology, see T1 space.
• A T-1 is also a training aircraft used by the United States Air Force.
• T1 relaxation time (or spin-lattice relaxation time), is a biological parameter that is used in MRIs to distinguish between tissue types.
• A type of subway car designed and built by Bombardier and runs on the TTC Subway lines.

(See T1 (subway car) )