Meaning

• (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."

Meaning

• To take a subset of.

• To extract only the portions of (a font) that are needed to display a particular document.