Set deeply below a surface, as on a face. / (figuratively) firmly in place; long-standing

(mathematical analysis) A subset of a given measurable space which is a member of the σ-algebra of that space.

A drum kit

(mathematical analysis) Any of the members of a Borel σ-algebra.

(mathematics) A set which contains any convex combination of any (finite) subset of its points.

Alternative form of setup

(carpentry) A punch that drives in a nail so that its head is level with or below the surface.

(US, idiomatic, countrified dialect) To sit down for a period of time, especially in the company of other people and in order to relax or to engage in casual conversation.

(mathematics) In complex dynamics, a set consisting of values generated by a mathematical function such that an arbitrarily small perturbation can cause drastic changes in the sequence of iterated function values.

(intransitive, idiomatic) To begin working. / (intransitive, idiomatic, with gerund) To begin working at. / (transitive) To cause to begin working.