(of a set S) A set A such that every element of A is also an element of S.
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.
A group of things or people, all of which are in a specified larger group.
We asked a subset of the population of the town for their opinion.
To take a subset of.
To extract only the portions of (a font) that are needed to display a particular document.