(set theory) Any one of the subsets into which an equivalence relation partitions a set, each of these subsets containing all the elements of the set that are equivalent under the equivalence relation.

occupying a position between the upper class and the working class

A social and economic class lying above the working class and below the upper class.

(law, chiefly US, Canada) A lawsuit brought by a single plaintiff or small group of plaintiffs as a representative of a large group of others having a common interest, with everyone in the large group eligible to receive damages if the plaintiff is successful.

of high social position / aristocratic

those people at the top of a social hierarchy / the aristocracy

An excellent performance. / Someone or something whose performance is excellent. / (sometimes sarcastically) Someone or something who is generally well-mannered, conscientious, etc.

Not of the elite, of the peasantry or working-class. / Unrefined.

(idiomatic) A student who frequently makes jokes or pokes fun; a wiseacre.

(role-playing games) One of a number of pre-determined groups into which characters in role-playing games are grouped, based on the focus of their skills.