1. On one occasion
   "once I ran into her";
   - one time, in one case
    
2. At a previous time
   "she was a dancer once";
   - formerly, at one time, erstwhile [archaic], erst [archaic], sometime [archaic]
    
3. As soon as
   "once we are home, we can rest";
   - when

Encyclopedia: Once, Twice, Three Times an Aisling