A colloquial expression meaning an extremely short time; equivalent to 'in an instant' or 'in the blink of an eye'.
気がついたら試合はあっちゅう間に終わっていた。
Before I knew it, the match was over in an instant.
子供の成長はあっちゅう間だね。
Children grow up in the blink of an eye, don't they?
準備に時間はかからなかった。あっちゅう間に終わったよ。
The preparations didn't take long. It was done in no time.
電車の乗り換えは案外スムーズで、あっちゅう間に次の電車に乗れた。
The train transfer was surprisingly smooth; I was on the next train in no time.