-- an acronym for Human/Computer Interaction
or Human/ Computer Interface
. The former describes a process, the latter describes a thing.
Human/Computer Interaction is a design process underlying the development of computer interfaces to match the requirements of the human/user. Subjects such as cognitive psychology, proxemics, semiotics, and symbology are synergistically applied to the design of the interface. Issues integral to HCI include user efficiency, user procedures, information organization, and task functionality.
Human/Computer Interface is the interface or link between the computer and human. Its development is derived from the design process of HCI.