1. a basis or cause, as for some belief, action, fact, event, etc.:  the reason for declaring war.
  2. a statement presented in justification or explanation of a belief or action:  I dare you to give me one good reason for quitting school!
  3. the mental powers concerned with forming conclusions, judgments, or inferences:  Effective leadership requires a person of reason.
  4. sound judgment; good sense.
  5. normal or sound powers of mind; sanity.
  6. Logic. a premise of an argument.
  7. Philosophy.
    1. the faculty or power of acquiring intellectual knowledge, either by direct understanding of first principles or by argument.
    2. the power of intelligent and dispassionate thought, or of conduct influenced by such thought.
    3. Kantianism. the faculty by which the ideas of pure reason are created.
verb (used without object)
    1. to think or argue in a logical manner.
    1. to form conclusions, judgments, or inferences from facts or premises.
  1. to urge reasons which should determine belief or action.
verb (used with object)
    1. to think through logically, as a problem (often followed by out).
    1. to conclude or infer.
    1. to convince, persuade, etc., by reasoning.
  1. to support with reasons.