1. great in importance or size
a monumental challenge/undertaking/problem/effort
Something monumental is significant in size or impact. A 'monumental challenge' for example is a huge challenge that cannot be easily overcome. A 'monumental mistake' is one that cannot be corrected easily and has huge consequences. Something 'monumental' may also serve as a monument. "That was a monumental case for the high court." 'Monumental' is an adjective that is used more often in formal speech or writing.