An adverb meaning 'very' or 'considerably'; used to indicate a fairly large degree or amount.
今日はずいぶん寒いから、コートを着て行ったほうがいいよ。
It's rather cold today, so you should wear a coat.
その店は値段がずいぶん高くなったね。
The prices at that shop have become quite high.
来週までにずいぶん宿題があるから、早めに始めよう。
There is a lot of homework due next week, so let's start early.
彼は日本語がずいぶん上手になったね。
He's gotten considerably better at Japanese.
(Archaic/strong) Used as a quasi-adjective to mean 'terrible' or 'reprehensible', often carrying contemptuous nuance.
あの男はずいぶんの振る舞いをしたと人々は言った。
People said that man behaved terribly.
ずいぶんな言い草だが、根拠はあるのか？
Those are harsh words — do you have any basis for them?
(Somewhat old-fashioned) Used as a quasi-adjective meaning 'remarkable' or 'extraordinary'.
彼女の演技はずいぶんな才能を感じさせる。
Her acting shows remarkable talent.
その発見はずいぶんな意義があると評価された。
That discovery was evaluated as having considerable significance.