A terminal adapter (TA) is a hardware interface between a computer and an Integrated Services Digital Network line. It's what replaces a modem when you are using an ISDN connection. Unlike "plain old telephone service," which carries signal in analog (voice) form between your computer and the telephone company's office, ISDN carries signals in digital form so there is no need to modulate and demodulate between analog and digital signals. The terminal adapter is what you have to install on a computer so that data can be fed directly into the ISDN line in digital form. Since ISDN service is not available from telephone companies in all areas, the terminal adapter is not usually built into a computer. You purchase and install it when you sign up for ISDN service.
Some manufacturers and telephone companies use the term ISDN modem, instead.