Asymmetric digital subscriber line (ADSL) is a type of digital subscriber line (DSL) technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. ADSL differs from the less common symmetric digital subscriber line (SDSL). In ADSL bandwidth and bit rate are said to be asymmetric, meaning greater toward the customer premisis ( downstream ) than the reverse ( upstream ).

Source: https://en.wikipedia.org/wiki/Digital_subscriber_line