The Free On-line Dictionary of Computing (30 December 2018):

User Network Interface

    (UNI) An interface point between
   ATM end users and a private ATM switch, or between a
   private ATM switch and the public carrier ATM network.

   The physical and protocol specifications for UNIs are
   defined by the ATM Forum's UNI documents, which allow for
   various types of physical interfaces.

   See also: NNI

   (1999-01-23)