A device that connects a computer to an external digital communications line, such as an ISDN line. A terminal adapter is a bit like a modem , but whereas a modem needs to convert between analog and digital signals, a terminal adapter only needs to pass along digital signals. As ISDN becomes more common, future computers will probably have terminal adapters built in. Currently, though, you need to purchase a separate terminal adapter if you want ISDN access. You can get an internal adapter or an external adapter than connects to your computer's serial port.