The Free On-line Dictionary of Computing (19 January 2023):

Terminal Adapter
TA
Terminal Adaptor

    (TA) Equipment used to adapt
   Integrated Services Digital Network (ISDN) Basic Rate
   Interface (BRI) channels to existing terminal equipment
   standards such as EIA-232 and V.35.  A Terminal Adapter is
   typically packaged like a modem, either as a stand-alone
   unit or as an interface card that plugs into a computer or
   other communications equipment (such as a router or PBX).
   A Terminal Adapter does not interoperate with a modem; it
   replaces it.

   [ISDN FAQ].

   (1994-10-03)