On purpose; deliberately; intentionally.
そんな失礼なことをわざとしたの？
Did you do something that rude on purpose?
彼は注意を引くためにわざと遅刻したらしい。
It seems he deliberately arrived late to get attention.
わざとじゃない、すみませんでした。
I didn't do it on purpose — I'm sorry.