1. very large, impressive, or significant in size or importance
2. relating to or serving as a monument
3. extremely great in degree; requiring enormous effort

Examples

• The construction of the cathedral was a monumental achievement.
• She made a monumental mistake by deleting all the files.
• The monumental statue honors fallen soldiers.
• Building the railroad required monumental effort from thousands of workers.
• The discovery had monumental implications for medical research.
• They erected a monumental arch to celebrate the victory.
• The task of cleaning up after the hurricane was monumental.