1 rare A chance occurrence or circumstance; (contextually) a fortuitous event, a mishap. In non-count use: fortuitous disposition, chance. Compare "accident".
2US Usually with capital initial. Preceded by a possessive, as a mock title of respect given to a person who has succeeded to an office (especially the presidency of the United States) by chance or mishap, such as the death of the previous incumbent. Chiefly in "His Accidency".
Mid 17th century; earliest use found in John Lightfoot (1602–1675), Hebraist and biblical scholar. From post-classical Latin accidentia unforeseen event: see -ency.