ADSL (Asymmetric Digital Subscriber Line) refers to both the technology and the equipment which allow high-speed digital communication, including video signals, across an ordinary twisted-pair copper phone line, with speed up to 9 megabits per second downstream to the customer and up to 800 kilobits per second (800 kbps) upstream.

ADSL is sometimes called Asymmetric Digital Subscriber Loop. It is a technology for transmitting digital information at high bandwidths on existing phone lines to homes and businesses. Unlike regular dialup phone service, ADSL provides continously-available, "always on" connection. ADSL is asymmetric in that it uses most of the channel to transmit downstream to the user and only a small part to receive information from the user. ADSL simultaneously accommodates analog (voice) information on the same line. ADSL is generally offered at downstream data rates from 512 Kbps to about 6 Mbps. A form of ADSL, known as Universal ADSL or G.Lite, has been initially approved as a standard by the ITU.

ADSL was specifically designed to exploit the one-way nature of most multimedia communication in which large amounts of information flow toward the user and only a small amount of interactive control information is returned. Several experiments with ADSL to real users began in 1996. In 1998, wide-scale installations began in several parts of the U.S. In 2000 and beyond, ADSL and other forms of DSL are expected to become generally available in urban areas. With ADSL (and other forms of DSL), telephone companies are competing with cable companies and their cable modem services.