Implicit interaction is a form of Human Computer Interaction where the user does not perform an explicit action or issue a command, but instead relies on the computer understanding things that user is doing naturally. In human--human communications, the tone of voice, a small gesture or a glance all carry meaning as well as explcit words or signals; implicit interaction takes the same approach for himan--computer communication. Thee term can also be used more broadly to include interaction techniques, such as incidental interaction, where the system is some ways observes the human's activity and makes inferences based on that.
Used on Chap. 19: page 459