1 Happening as a minor accompaniment to something else.
1.1 Occurring by chance in connection with something else.
2 incidental to – Happening as a result of (an activity)
An incidental expense, event, etc.
Early 17th century originally from medieval Latin incidentalis, from Latin incident- ‘falling upon, happening to’ (from the verb incidere).
The illustrations added to these definitions are incidental, in spite of their frequent use.