Stands for "Asymmetric Digital Subscriber Line." ADSL is a type of DSL, which is a method of transferring data over copper telephone lines. While symmetrical DSL (SDSL) uploads and downloads data at the same speed, ADSL has different maximum data transfer rates for uploading and downloading data.
For example, an ADSL connection may allow download rates of 1.5Mbps, while upload speeds may only reach 256Kbps. Since most users download much more data than they upload, this difference usually does not make a noticeable impact on Internet access speeds. However, for Web servers or other computers that send a lot of data upstream, ADSL would be an inefficient choice.