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):

interface
    n 1: (chemistry) a surface forming a common boundary between two
         things (two objects or liquids or chemical phases)
    2: (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]
    3: the overlap where two theories or phenomena affect each other
       or have links with each other; "the interface between
       chemistry and biology"
    4: (computer science) computer circuit consisting of the
       hardware and associated circuitry that links one device with
       another (especially a computer and a hard disk drive or other
       peripherals) [syn: interface, port]

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

interface

    A boundary across which two systems communicate.  An
   interface might be a hardware connector used to link to other
   devices, or it might be a convention used to allow
   communication between two software systems.  Often there is
   some intermediate component between the two systems which
   connects their interfaces together.  For example, two EIA-232
   interfaces connected via a serial cable.

   See also graphical user interface, Application Program
   Interface.

   (1996-05-22)