A person of major significance, accomplishment or acclaim.
“Newton and Einstein are two of the greats of the history of science.”
(in combinations such as "two-greats", "three-greats" etc.) An instance of the word "great" signifying an additional generation in phrases expressing family relationships.
“My three-greats grandmother.”
adjective
Relatively large in scale, size, extent, number (i.e. having many parts or members) or duration (i.e. relatively long); very big.
“A great storm is approaching our shores.”
(qualifying nouns of family relationship) Involving more generations than the qualified word implies — as many extra generations as repetitions of the word great (from 1510s). [see Derived terms]