momentous

tärkeä outstanding Outstanding in importance, of great consequence.
1725, w:Daniel Defoe|Daniel Defoe, Everybodys Business is Nobody's Business'':

The reason why I did not publish this book till the end of the last sessions of parliament was, because I did not care to interfere with more momentous affairs.

"It has been a momentous month, and I hope we shall all retain healthful recollections of it as long as we live."

What to the other parties was merely the sale of a ship was to him a momentous event involving a radically new view of existence.

Natural selection is arguably the most momentous idea ever to occur to a human mind, because it — alone as far as we know — explains the elegant illusion of design that pervades the living kingdoms and explains, in passing, us.