as a noun, 'great' refers to a person who is eminent in their field.
Newton and Einstein are among the greats of science.
She is considered one of the greats in classical music.
The museum had works from several of the greats.
When used as a noun, 'great' is often pluralized and usually refers to people who are exceptional in their field.
as an adjective, 'great' is used to describe something that is above average in size or amount, or remarkable in quality or degree.
She had a great time at the party.
That's a great idea!
He made a great contribution to science.
'Great' is often used for emphasis in everyday speech but can be considered vague in formal writing. The word can sometimes be used ironically to mean the opposite, depending on context and tone.
as an adverb, 'great' is used informally to emphasize an adjective or another adverb.
She did great well on her exams.
The team played great today.
You did that great quickly.
the adverbial use of 'great' is generally considered informal and may not be suitable for formal or academic writing. Avoid using 'great' as an adverb in formal contexts; use 'greatly' instead.