Or do you mean ADSL, the broadband internet connection?
If so, it's Asymmetric digital subscriber line.
Line, because it's, you know.. a line. Subscriber because it's a subscription service that you pay an amount monthly for, not a pay as you go type thing. Digital because the signals it uses are digital, not analogue like the phone system. And asymmetric because the upload and download speeds are different (for example, 2mbps download, 256kbps upload).