"An interface is a point of interaction between two systems or work groups" 
"In the industrial design field of human-machine interaction, the user interface is (a place) where interaction between humans and machines occurs. The goal of interaction between a human and a machine at the user interface is effective operation and control of the machine, and feedback from the machine which aids the operator in making operational decisions. Examples of this broad concept of user interfaces include the interactive aspects of computer operating systems, hand tools, heavy machinery operator controls. and process controls. The design considerations applicable when creating user interfaces are related to or involve such disciplines as ergonomics and psychology"
"A user interface is the system by which people (users) interact with a machine. The user interface includes hardware (physical) and software (logical) components. User interfaces exist for various systems, and provide a means of: Input, allowing the users to manipulate a system, and/or Output, allowing the system to indicate the effects of the users' manipulation" .
Social interface is a concept from social science (particularly, sociology of technology). It can be approached from a theoretical or a practical perspective. As a concept of social interface theory, social interface is defined by Long (1989, 2001). In 2001 his revised definition was:
"A social interface is a critical point of intersection between different lifeworlds, social fields or levels of social organization, where social discontinuities based upon discrepancies in values, interests, knowledges and power, are most likely to be located".
Long, Norman, ed. 1989. Encounters at the Interface: a Perspective in Social Discontinuities in Rural Development, Wageningse Sociologische Studies 27. Wageningen: Wageningen Agricultural University. For new 2001 definition, see Print, p.243 in Norman Long, Development Sociology: Actor Perspectives, Routledge, 2001, ISBN 0415235359
In other words, interfaces are the areas in which social friction can be experienced and. Where diffusion of new technology is leading to structural discontinuities (which can be both positive or negative), the interface is where they will occur" .
Interface (computer science)
"In the field of computer science, an interface refers to a point of interaction between components, and is applicable at the level of both hardware and software. This allows a component, whether a piece of hardware such as a graphics card or a piece of software such as an internet browser, to function independently while using interfaces to communicate with other components via an input/output system and an associated protocol" .