From Longman Dictionary of Contemporary Englishat onceat oncea)IMMEDIATELYimmediately or without delayNow, go upstairs at once and clean your room!When I saw him I recognized him at once.b)TIME/AT THE SAME TIMEtogether, at the same timeI can’t do two things at once!Don’t all talk at once.RegisterIn everyday English, when people mean 'immediately', they usually say right away or, in British English, straight away rather than at once:I recognized him right away. →once
Examples from the Corpus
at once• I can't do two things at once!• I can't understand what you're saying when you both talk at once.• You're trying to do too many things at once.• Everyone knew at once how serious the situation was.• Anyone know the answer? Don't all shoutat once, put your hand up.• You can't have three weeks' holiday all at once, you'll have to take them separately.