Good luck or fortune; a favorable chance or circumstance (can be used as a noun or na-adjective).
今日の試験で満点を取れたのは幸運だった。
It was lucky to get a perfect score on today's exam.
旅行中に晴れてくれて幸運だった。
We were fortunate that it cleared up during the trip.
その知らせを聞けたのは幸運と言えるだろう。
You could say it was fortunate to have heard that news.