1 [usually before noun]
a monumental achievement, piece of work, etc is very important and is usually based on many years of work:
a monumental contribution to the field of medicine
Charles Darwin's monumental study, 'The Origin of Species'
2 [only before noun]
extremely large, bad, good, impressive etc:
Banks and building societies were yesterday accused of monumental incompetence.
a monumental task
There was a monumental traffic jam on the freeway.
3 [only before noun]AA
relating to a monument or built as a monument:
a monumental arch