Deductive arguments, which are such that if the premisses are true the conclusion must be true.
Traditional logic studies the validity of syllogistic arguments.
Modern logic, more generally, identifies as valid those arguments which accord with truth-preserving rules. Any argument is valid if and only if the set consisting of its premisses and the negation of its conclusion is inconsistent. Propositions which are semantically valid, i.e. are true under any alternative interpretation of the non-logical words. The Oxford Companion to Philosophy 2. The Oxford Companion to Philosophy 4.