Human-computer interaction (HCI) is research in the design and the use of computer technology, which focuses on the interfaces between people (users) and computers. HCI researchers observe the ways humans interact with computers and design technologies that allow humans to interact with computers in novel ways. A device that allows interaction between human being and a computer is known as a "Human-computer Interface (HCI)."
Human-computer interaction, Wikipedia
I believe that HCI is the field of research and development focused on improving communication between people and machines to enhance people's work and quality of life.
Interface devices play a significant role in human-computer communication. Moreover, these devices have the capability to introduce people to a new world, such as mouse, which opened graphical user interface (GUI) world.
Picture, Douglas Engelbart's mouse, the very first computer mouse, invented by Douglas Engelbart and designed by Bill English based on Engelbart's sketches.