momentous

adj

1. Outstanding in importance, of great consequence.

   “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.”

   “The last train ran on December 31, 1932, almost unnoticed, for it coincided with the running of the last steam passenger train down the main line from London to Brighton, a much more momentous event.”

   “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.”