The Collaborative International Dictionary of English v.0.48:

interface \interface\ n.
   1. a surface forming a common boundary between two things,
      especially between two fluids. [WordNet sense 1]
      [WordNet 1.5]

   2. (Computers) hardware that links one device with another
      (especially a computer). [WordNet sense 2]
      [WordNet 1.5]

   3. (Computers) That part of a computer program which controls
      the way a program interacts with a user; the manner of
      inputting and outputting of data, and the way information
      is presented on a computer monitor; also called user
      interface; as, a graphical user interface; a
      character-based interface.
      [PJC]

WordNet (r) 3.0 (2006):

user interface
    n 1: (computer science) a program that controls a display for
         the user (usually on a computer monitor) and that allows
         the user to interact with the system [syn: interface,
         user interface]

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

user interface

   (UI) The aspects of a computer system or program which can be
   seen (or heard or otherwise perceived) by the human user, and
   the commands and mechanisms the user uses to control its
   operation and input data.

   A graphical user interface emphasises the use of pictures
   for output and a pointing device such as a mouse for input
   and control whereas a command line interface requires the
   user to type textual commands and input at a keyboard and
   produces a single stream of text as output.

   A user interface contrasts with, but is typically built on top
   of, an Application Program Interface (API).

   See also user interface copyright.

   (1995-02-20)