introduction > personal views

summary		 In order to write about interfaces, one has to know what an interface is, exactly. A basic requirement, sure, but hardly a simple one. As it seems, there is no clear, generally accepted, definition of what an interface is. There are literally hundreds of definitions circulating, all vaguely pointing in the same direction, but without a really unified core.
First of all, the term 'interface' is extremely broad and open to interpretation. 'Interface' is basically a homonym, with a lot of different meanings in a lot of different contexts. It is used in every scientific field where human-machine interaction is a subject. Its origins lie even further, as the following excerpt from The New Shorter Oxford English Dictionary [3] illustrates:
  1. A surface at which two portions of matter or space meet (ca.1870-1899)
  2. A means or place of interaction between two systems, organisations, etc.; a meeting point or common ground between two parties, disciplines, etc. (ca.1870-1899)

This is the basic definition that covers the essential meaning of 'interface'. It reaches beyond human-computer interaction, it covers basically any interaction or contact between two parties. It could just as easily be applied to the interaction between driver and car as to readers and books or viewers and TV's. It has no false limitations, but instead gives the term 'interface' a tangible meaning.
This definition also indicates that interfaces are always between two different environments. There is no such thing as the interface of a computer program, it is always between the software and the user. This definition tells interface designers once and for all that it takes two to interface.
The Oxford Dictionary also uses the phrase 'common boundary', which, in turn, is very important. An interface encompasses some sort of common ground, something that is mutually understood. In computer interfaces, an example can easily be found in the evolution from command-line interface to the desktop metaphor we know today. In the case of the command-line, the common ground was language. The user could more or less understand the words and phrases the computer displayed, and the computer could interpret the text input from the user. In today's desktop metaphors, the common ground has broadened a lot. In addition to language, we can now communicate with computers through more visual means, like icons and windows. The computer knows which icon is associated with which file type, how many windows are open and how they are stacked, and so do we, the user.
Steven Johnson, in his book 'Interface Culture' defines 'interface' as 'software that shapes the interaction between user and computer, … a kind of translator' [1]. Note that he uses the terms 'software' and 'computer', limiting the possible applications for his definition. This definition implies that interfaces are always pieces of software running on a computer, used for facilitating interaction with the computer. Throughout his book, Stephen Johnson maintains the emphasis on computer interfaces. He focuses mainly on computer interfaces in a historical perspective and provides a possible scenario for the future.
In Nicholas Negroponte's 'Being Digital' [2], no definition is even mentioned, although the book contains an entire chapter titled 'Interface'. One might call his subtitle for this chapter a definition, 'Where people and bits meet', but it would be a very vague one.