An adverbial phrase meaning in a very short time, in an instant, or before you know it.
気づいたら試験が終わり、時間があっという間に過ぎていた。
Before I knew it the exam was over and time had flown by.
子どもはあっという間に大きくなるから、写真をたくさん残しておきたい。
Children grow up in no time, so I want to keep lots of photos.
準備に時間がかかるかと思ったが、案外あっという間に終わった。
I thought the preparation would take time, but surprisingly it finished quickly.