Definition of "momentous"

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.

1725, Daniel Defoe, Everybody's Business is Nobody's Business

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

1831, James Fenimore Cooper, chapter 31, in Homeward Bound

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.

1902, Joseph Conrad, chapter 3, in The End of the Tether

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.

2007 July 1, Richard Dawkins, “Inferior Design”, in New York Times, retrieved 19 November 2013