adverb (r)

• without advance planning(adv.all)

  Example:
  They met accidentally.
  

  Synonym:
  by chance, circumstantially, unexpectedly
  Antonym:
  advisedly, by choice, by design, deliberately, designedly, intentionally, on purpose, purposely
  

  source: wordnet30
• of a minor or subordinate nature(adv.all)

  Synonym:
  incidentally
  

  source: wordnet30
• without intention; in an unintentional manner(adv.all)

  Synonym:
  unintentionally
  

  source: wordnet30
• In an accidental manner; unexpectedly; by chance; unintentionally; casually; fortuitously; not essentially.(adverb)

  source: webster1913