Stands for "Asymmetric Digital Subscriber Line."
ADSL is a type of DSL Internet connection. Like SDSL, it transfers digital data over copper telephone lines. The word "asymmetric" means that the upload and download bandwidth are different; the download speed offered to the user is higher than the upload speed.
An ADSL Internet connection is asymmetrical because a copper telephone line only offers a limited amount of bandwidth. Upload and download traffic occupy different parts of the frequency spectrum transmitted over the lines, with the ISP determining the part of the spectrum allocated to each. Since most home Internet users download a lot more data than they upload, ISPs allocate more of that spectrum to download speeds than to upload speeds.
An SDSL line, on the other hand, splits the available spectrum evenly. This provides a symmetrical connection with equal upload and download speeds, but this also means that the maximum download speed of an SDSL line is lower than that of an ADSL line.