A user interface describes any mechanism by which a human interacts with a machine or computer. From automobiles to toasters to personal computers, nearly every mechanical and electrical device has a user interface. Generally, the term refers to the method by which a person interacts with a computer. In this area, a user interface may not simply refer to the mechanism of interaction, but the program that allows it as well.
The term user interface originated in industrial machinery. It originally referred to the area in which a human operator made the industrial machine operate. It didn't matter whether the machine was a complicated lift or a simple hand drill. Eventually, the term migrated into other areas of manufacturing and materials processing. When home electronics began to appear on the market, the term came into people’s homes with them.
The wide spread use of the term allowed it to move into even more technical fields. Tools, electronic measuring devices and even steering wheels used the idea. Computers provided the biggest new addition to the user interface term. Computer designers adopted the phrase for their own use around the time home computers were becoming more of a possibility during the mid-1960s.
As personal computers became more common, the term began to drop out of other fields. User interface became so closely associated with computer interfaces that older uses had to pick up new methods of describing their interactions. The two most common terms for the industrial world are man-machine interface and human-machine interface. In processing and manufacturing, operator interface took over. In other areas, the term simply fell out of use without a replacement.
In computer science, a user interface plays a slightly bigger role than in other fields. Originally, it just had to take in requests and output responses. In the case of large machines, that may have been as simple as pushing a button that made soup drop into a can. With computers, the user interface also had to allow the input of new commands. It had to give the computer the ability to do more than what it could do before.
This increase in meaning expanded even more with the advent of the graphical user interface. These computer interfaces use images and advanced selection methods, like computer mice, to streamline the interaction between a person and a computer. The term expanded again to cover the entire program used as an interface. Now, things that weren’t directly related to human interaction were still part of the term.