in an instant; in a flash; in no time at all
Describes something happening so quickly that it feels almost instantaneous. Often used when an event or change occurs before one realizes it.
Summer vacation was over in the blink of an eye.
He solved the problem in no time at all.
あっという間に stresses how surprisingly short the time was, while みるみる highlights watching the visible change unfold.
Standard kana-kanji mixed spelling; the most common written form.
Variant with 言う in kanji; less common but still seen.
Derived from the phrase あっと言う間 (the time it takes to say 'ah'), with the adverbial particle に. The exact historical origin is uncertain, but it is a natural colloquial expression for a very short time.