In telecommunications, a terminal adapter is an interfacing device employed at the "R" reference point in an ISDN environment that allows a non-ISDN terminal at the physical layer to communicate with an ISDN network.
It performs a similar role for ISDN to that which a modem performs for the PSTN.
Note: Typically, a terminal adapter will support standard RJ-11 (or other country-specific) telephone connection plugs for voice and RS-232C, V.35 and RS-449 interfaces for data.
Source: from Federal Standard 1037C
