The ACM SIGCHI, the Special Interest Group for Computer/Human Interaction, define as such:
Human-computer interaction is a discipline concerned with the design, evaluation and implementation of interactive computing systems for human use and with the study of major phenomena surrounding them.
HCI (human-computer interaction) is the study of how people interact with computers and to what extent computers are or are not developed for successful interaction with human beings. A significant number of major corporations and academic institutions now study HCI. Historically and with some exceptions, computer system developers have not paid much attention to computer ease-of-use. Many computer users today would argue that computer makers are still not paying enough attention to making their products usable. However, computer system developers might argue that computers are extremely complex products to design and make and that the demand for the services that computers can provide has always outdriven the demand for ease-of-use.
Please do not say 'user friendly'. Friendly is a strange concept that doesn't translate international boundaries well. A good system shouldn't necessarily be 'friendly' or intrusive. The correct term is "usable".
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