The user interface is the part of a computer system that a user sees, hears or otherwise difrectly enagges with. The field of Human Computer Interaction includes aspects targeted at improving the user experience of using user interfaces, and the software needed to create them.
Used on Chap. 13: page 290; Chap. 18: page 425; Chap. 19: page 481; Chap. 20: page 489