Incidental interaction refers to cases where the user's focus is on performing one task (the sensed task), but data gathered about it incidentally helps another task (the supported task). For example, smart glasses might observe the food you are eating and use it to create a food diary. Incidental interaction is at one extreme of the intention spectrum.
Used in Chap. 4: page 60
Used in glossary entries: intention continuum, sensed task, supported task