Collections (4)
'collections' is the definition.
(set is a kind of collection)
This is all the clue.
(Other definitions for sets that I've seen before include "Classes" , "Groups of things of the same kind" , "Major divisions of tennis match" , "Plants" , "Scenery etc. of dramatic productions" .)
'collections' is the definition.
('aggregation' can be a synonym of 'collection')
This is the entire clue.