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]

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "interface":
   ankle, articulation, border line, bound, boundary,
   boundary condition, boundary line, bourn, break boundary,
   breakoff point, butt, ceiling, cervix, circumscription, clinch,
   closure, compass, confine, connecting link, connecting rod,
   connection, coupling, cutoff, cutoff point, deadline, delimitation,
   determinant, division line, dovetail, elbow, embrace, end,
   extremity, finish, floor, frontier, gliding joint, hedge,
   high-water mark, hinge, hinged joint, hip, join, joining, joint,
   juncture, knee, knuckle, limen, limit, limitation, limiting factor,
   line, line of demarcation, link, low-water mark, lower limit,
   march, mark, mete, miter, mortise, neck, pivot, pivot joint,
   rabbet, scarf, seam, shoulder, start, starting line,
   starting point, stitch, suture, symphysis, target date, term,
   terminal date, terminus, threshold, tie rod, time allotment,
   toggle, toggle joint, union, upper limit, weld, wrist



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

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)