(all) at once; at the same time; in one go; in one sitting.
いっぺんに全部食べるのは無理だ。
I can't eat it all at once.
書類をいっぺんに片付けてしまった。
I took care of all the paperwork in one go.
いっぺんに二つのボタンを押してしまった。
I pressed two buttons at the same time.
right away; in an instant; suddenly.
彼の顔を見ると、いっぺんに元気が出た。
Seeing his face instantly lifted my spirits.
驚いて声がいっぺんに出なかった。
I was so surprised I couldn't speak right away.