at once in WordNet English dictionary

1. simultaneously
   "he took three cookies at a time"
   at a time  at once  at one time  at a time  at once  at one time 
   
2. without delay or hesitation; with no time intervening
   "he answered immediately"; "found an answer straightaway"; "an official accused of dishonesty should be suspended forthwith"; "Come here now!"
   immediately  instantly  straightaway  straight off  directly  now  right away  at once  forthwith  like a shot  immediately  instantly  straightaway  straight off  directly  now  right away  at once  forthwith  like a shot 
   

at once in English Wiktionary