Something mysterious, strange, or marvelous; a sense of wonder or oddity. Can mean wonderfully strange as well.
不思議な夢を見て、朝から気持ちが落ち着かない。
I had a strange dream and haven't felt calm since morning.
彼女の回復は不思議に思えるほど早かった。
Her recovery was surprisingly quick, almost miraculous.
子どもは不思議そうに空を見上げた。
The child looked up at the sky with wonder.
Used adverbially (不思議と) to mean 'strangely enough' or 'for some reason', expressing an unexpected feeling or result.
不思議と緊張しなかった。
Strangely enough, I wasn't nervous.
不思議と彼のことを思い出した。
Oddly enough, I remembered him.
不思議と天気予報が外れて、晴れた。
Strangely, the weather forecast was wrong and it turned out sunny.