Any type of modem used for synchronous transmission of data over circuit-switched digital lines.
One example of a digital modem is an ISDN terminal adapter. Digital modems are not used for changing analog signals into digital signals because they operate on end-to-end digital services.
Instead, they use advanced digital modulation techniques for changing data frames from a network into a format suitable for transmission over a digital line such as an Integrated Services Digital Network (ISDN) line. They are basically data framing devices, rather than signal modulators.