WordWeb Online

1. Without advance planning
   "they met accidentally"
   - by chance, circumstantially, unexpectedly
    
2. Of a minor or subordinate nature
   "these magnificent achievements were only accidentally influenced by Oriental models"
   - incidentally
    
3. Without intention; in an unintentional manner
   "she hit him accidentally"
   - unintentionally

See also: accidental

Antonym: deliberately

Encyclopedia: Accidentally