Real Dictionary

great
You can describe someone who is successful and famous for their actions, knowledge, or skill as great.
You use great to describe something that is very large. Great is more formal than big.
adjective: Great means large in amount or degree.
adjective: You use great to describe something that is important, famous, or exciting.
plural noun: The greats in a particular subject or field of activity are the people who have been most successful or famous in it.

great in American English
of much more than ordinary size, extent, volume, etc.

great in American Englishesp collectively
adjective: unusually or comparatively large in size or dimensions

great in British English
adjective: relatively large in size or extent; big

great-
prefix: Great- is used before some nouns that refer to relatives. Nouns formed in this way refer to a relative who is a further generation away from you. For example, your great-aunt is the aunt of one of your parents.

great- in American English
older (or younger) by one generation

great- in British English
prefix: being the parent of a person's grandparent (in the combinations great-grandfather, great-grandmother, great-grandparent)