Stands for "Transmission System Level 1."
A T1 line is a telecommunications line that supports 24 voice channels when used for telephone service, or a total bandwidth of 1.544 Mbps when used as an Internet connection. T1 lines typically operate over twisted pair copper wire, similar to a telephone wire, but can also run over coax cable or fiber optic cable. T1 lines are also known as DS1 (Digital Signal Level 1) lines.
Telecoms first deployed T1 lines in the 1960s for the digitized transmission of multiple telephone lines over a single connection, known as a trunk line, between telephone switching centers. Telecoms later offered dedicated T1 lines to businesses, supplying up to 24 phone lines over a single connection. Eventually, T1 lines provided those businesses with constant, reliable Internet access. Customers that needed more bandwidth could purchase a bonded T1 line, offering twice the bandwidth of a single T1.
As other forms of high-speed internet became available, T1 lines declined in popularity. Some ISPs who offered T1 lines for decades are discontinuing them as part of a general retirement of copper wire networks.
NOTE: While 1.544 Mbps might not seem like a fast Internet connection now, a T1 line was a high-speed connection compared to the slow dial-up speeds of the 1980s and 1990s. It was also a dedicated, always-on connection that was not prone to dropping out when a phone call came in.