incidental /-dentˈl/ adjective

1. Incident
2. Striking or impinging
3. Liable to occur
4. Naturally attached
5. Accompanying
6. Concomitant
7. Occasional, casual

Anything that occurs incidentally

incidentˈally adverb

1. In an incidental way
2. (loosely) by the way, parenthetically, as a digression

incidentˈalness noun

incidental music noun

Music accompanying the action of a play, film, television programme, etc

incident centre, incident office or incident room noun

A temporary establishment set up near the scene of a crime, disaster or unrest, etc by the police or army in order to collect information and monitor the situation

angle of incidence (physics)

The angle between an incident ray and the normal to the surface it falls on