means A set whose every member also belongs to another, larger set.

from A modern compound stitched together from Latin parts: the prefix 'sub-' meaning 'under' or 'below,' and 'set,' the English noun for a collection of things. The mathematical sense — a set contained within another — is a 19th- and 20th-century coinage that grew up alongside set theory itself, when mathematicians needed tidy words for the ways one collection can nest inside another.