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

Antonym: advisedly

Encyclopedia: Accidentally