If you refer to a decision, event, or change as momentous, you mean that it is very important, often because of the effects that it will have in the future.
例句
...the momentous decision to send in the troops...派遣部队的重大决定The past three years have been among the most momentous in world history.过去的3年是世界历史上最重要的时期之一。
英汉词典释义
adj.
重大的, 严重的
The momentous news was that war had begun.重大的新闻是战争已经开始。
英英词典释义
Adjective
1. of very great significance;
"deciding to drop the atom bomb was a very big decision""a momentous event"