adjective

1. Accidental; as the result of some unforeseen happening or series of events; unexpected; resulting from an event without the foresight, expectation, aid, or design of the person by whose agency it was caused; unintentional.

   “That the injury was by accident is not questioned.”

Synonyms: causeless, incidental, unintended

adverb

1. Accidentally; unintentionally; without meaning to; as the result of an event not expected, aided, or willingly planned.

   “Sorry. I knocked over the vase by accident.”

2. As the result of some undesigned, unintended, unrelated, natural, or random occurrence; as the unexpected and unforeseen result of formerly unrelated factors.

   “The car got totaled by accident; a giant snowdrift fell off the church roof.”

3. By chance; unexpectedly; as the result of an unrelated series of events.

   “I hadn't seen her for years, but I ran into her by accident at the store.”