noun a set whose members are members of another set.
Examples
“The set \lbrace a, b\rbrace is a both a subset and a proper subset of \lbrace a, b, c\rbrace while the set \lbrace a, b, c\rbrace is a subset of \lbrace a, b, c\rbrace but not a proper subset of \lbrace a, b, c\rbrace.”
“We asked a subset of the population of the town for their opinion.”