From the second edition (1989):
interface, n.
(ˈɪntəfeɪs) [f. inter- 2b + face.]

1. A surface lying between two portions of matter or space, and forming their common boundary.

1882 Bottomley Hydrost. 13 The term interface denotes a face of separation, plane or curved, between two contiguous portions of the same substance. 1883 G. Chrystal in Encycl. Brit. XV. 264/1 The interface of the two liquids in the axial line.

2. transf. and fig. a. A means or place of interaction between two systems, organizations, etc.; a meeting-point or common ground between two parties, systems, or disciplines; also, interaction, liaison, dialogue.

1962 M. McLuhan Gutenberg Galaxy 141 (heading) The interface of the Renaissance was the meeting of medieval pluralism and modern homogeneity and mechanism. 1962 Evening Star (Washington, D.C.) 18 Aug. 1/6 Interface‥seems to mean the liaison between two different agencies that may be working on the same project. Ibid. 1/7 The Defense Communications Agency‥was made responsible for the resolution of interface problems. 1964 A. Battersby Network Analysis viii. 116 Interfaces: events should be established at stages where the work passes from one department to another—these stages are known as interfaces. 1965 H. I. Ansoff Corporate Strategy (1968) vi. 107 Functional organizations, such as research, development, finance, and marketing, have a strong interface with the outside environment. 1965 Internat. Sci. & Technol. Oct. 30/1 The advantages of high-speed transport were piddled away at the nodes or interfaces: from bus to train, train to train, city terminal to airport terminal, check-in counter to loading gate, and so on. 1967 Technology Week 23 Jan. 75/1 The interface across which the engineer-scientist and the biologist can interact is a broad one. 1967 Times Rev. Industry Feb. 27/1 The third interface between government and the marketing system is with the intermediate firm supplying either other intermediate firms or the consumer. 1967 Economist 16 Sept. p. ix/1 The North Sea and Channel ports form the biggest frontiers in world trade—or the biggest interface, in the language of the modern transport man, meaning the place where the greatest quantity of international cargo changes its mode of transport. 1970 Nature 23 May 684/1 The interface between physics and music is of direct relevance to‥the psychological effects of hearing. 1970 Interior Design Dec. 767/4 Educationalists are convinced that the need for the interface of lecturer and student will not diminish. 1972 Sci. Amer. Nov. 51/3 The issue of insanity as a defense in criminal cases‥is at the interface of medicine, law and ethics.

b. (An) apparatus designed to connect two scientific instruments, devices, etc., so that they can be operated jointly.

1964 Ann. N.Y. Acad. Sci. CXV. 574 The collection of components which connects the analog and digital computers to each other, and which controls and converts the data, is generally termed the ‘interface’. 1966 Electronics 3 Oct. 130 If a flight carries special equipment, then modular interfaces can easily be designed to adapt the general-purpose computer to the equipment. 1973 T. Allbeury Choice of Enemies xvi. 79 Programs are written in a computer language.‥ If you wanted to use one of the IBM languages on an ICL machine, you'd have to have what's called an interface to make the two different things compatible. 1973 Physics Bull. Apr. 242/3 Scobie and Wellum‥have built interfaces for two pulse height analysers.