To a great extent or degree.
"The new policy has greatly improved our office's working environment by reducing unnecessary meetings."
In plain English: Greatly means to do something with a lot of effort or intensity.
"The rain greatly improved the air quality in the city."
Usage: Use greatly to modify verbs, adjectives, or other adverbs to indicate a high degree or intensity of an action or quality. It is commonly paired with positive emotions like appreciated or admired, as in "greatly enjoyed," rather than describing physical dimensions.
Greatly comes from the Middle English word gretly, which was formed by adding the suffix -ly to great. The term has maintained its original meaning of "to a large degree" as it traveled into modern English usage.