A method of reasoning or logical inference going from the general to the particular (or from cause to effect). In another respect, and according to Karl R. Popper (1902-1994), it involves stating a theory first and then trying to find facts to reject it. Darwin and Piaget are usually offered as prime examples of using the deductive method in arriving at their respective theories.
See Abstract reasoning, Deductive-nomological (D-N) model, Hypothetico-deductive method, Induction (philosophy), Logical reasoning, Reasoning (psychology), Reasoning (genre theory)