An adverb meaning 'surprisingly' or 'to an astonishing degree', used in the pattern '驚くほど〜だ' to express an unexpectedly high degree.
値段が驚くほど安かった。
The price was surprisingly cheap.
彼の回復は驚くほど早かった。
His recovery was astonishingly fast.
そのソフトは驚くほど使いやすい。
That software is amazingly easy to use.