x (for 'any') Digital Subscriber Line - There are many different types or 'flavors' of Digital Subscriber Line (DSL) services. To denote a specific type a letter is prefixed to the DSL acronym. For example, ADSL is Asymetrcial Digital Subscriber Line and SDSL is Symetrical Digital Subscriber Line.. When talking about DSL in general terms, it is usually referred to as simply "DSL" but is sometimes referred to as "xDSL" (where the 'x' denotes 'any' type letter).