De Morgan's laws

(used with a plural verb) Logic. two laws, one stating that the denial of the conjunction of a class of propositions is equivalent to the disjunction of the denials of a proposition, and the other stating that the denial of the disjunction of a class of propositions is equivalent to the conjunction of the denials of the propositions.

(used with a singular verb) Mathematics. the theorem of set theory that states that the complement of the union of two sets is equal to the intersection of the complements of the sets and that the complement of the intersection of two sets is equal to the union of the complements of the sets.