at once meaning

Adverb: at once

1. Without delay or hesitation; with no time intervening
   - immediately, instantly, straightaway, straight off, directly, now, right away, forthwith, like a shot 
2. Simultaneously
   - at a time, at one time

[American idiom]
immediately; at this very moment.
• John, come here at once!
• Bring me my coffee at once!
• Shall I do it at once or wait until morning?

[American slang]
immediately; right now.
  We must leave at once!
  You must come here at once; it is an emergency.

Examples

More:   Next

1. the matter is urgent and must be dealt with at once.
2. a strong objection at once occurred to us.
3. we must evacuate those soldiers at once!
4. he realized at once what was happening.
5. it is expedient that he should go away at once!