Greatly means to a large degree or by a large amount. When something changes greatly, it changes a lot. When you greatly admire someone, you don't just like them a little: you respect and look up to them deeply.

The word adds weight and significance to whatever it describes. A student who greatly improves in math doesn't just get slightly better; they make a major leap forward. A community greatly affected by a storm has experienced serious impact, not minor inconvenience.

You'll often see greatly in formal writing or serious contexts. Someone might say “I would greatly appreciate your help” when making an important request, rather than just “thanks.” Scientists might say a discovery greatly advances our understanding of the universe.

Greatly turns ordinary descriptions into emphatic ones. The difference between “this helped” and “this greatly helped” is the difference between a small favor and significant assistance. When you want to show that something matters a lot or changes things substantially, greatly is the word that conveys that importance.