expression, adverb
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.
あっという間に夏休みが終わってしまった。
Summer vacation was over in the blink of an eye.
彼はあっという間にその問題を解いた。
He solved the problem in no time at all.
すぐに simply means 'immediately' or 'soon', while あっという間に emphasizes the subjective feeling that something happened surprisingly fast.
瞬く間に is a more literary synonym meaning 'in the twinkling of an eye', whereas あっという間に is more colloquial and common in everyday speech.
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.