Naked DSL is DSL (Digital Subscriber Line) service in which the customer is not provided with dial tone. In regular DSL, a wire is run from the telephone switch to a piece of equipment called a Plain Old Telephone Service (POTS) splitter. The POTS splitter separates the DSL and voice bands and then a wire carrying both services runs from the splitter to the cable head, where it continues on to the customer on outside plant.
In naked DSL the portion of cable from the switch to the splitter is removed, therefore removing dial tone from the line. It is possible that the customer would have dial tone through a Competitive Local Exchange Carrier (CLEC) instead of the local telco (RBOC), or that they use cellular service exclusively in place of a traditional land line.