(set theory, of a set S) The set whose elements comprise all the subsets of S (including the empty set and S itself).

(set theory) The unique set that contains no elements, denoted ∅ or {}.

(mathematics, set theory, measure theory) A set that is small enough that it can be ignored for some purpose.

(mathematics, measure theory) A negligible set; set of zero measure. / (set theory) The empty set.

(set theory) A set large enough to contain all sets under consideration in the current context.

(mathematics) Of, relating to or using set theory.

something that is set aside for a specific purpose / (Britain and Europe) land that has been taken out of agricultural production to reduce crop surpluses

assigned for some purpose. / taken out of agricultural production / reserved or booked in advance

(transitive) To separate and reserve something for a specific purpose. / (transitive) To leave out of account; to omit or neglect. / (transitive) To disagree with something and reject or overturn it.

To initiate or begin some action. / To attack.