1 a form of reasoning in which a conclusion is drawn from two given or assumed propositions (premisses): a common or middle term is present in the two premisses but not in the conclusion, which may be invalid (e.g. all trains are long; some buses are long; therefore some buses are trains: the common term is long).
2 deductive reasoning as distinct from induction.
Derivatives:
syllogistic adj. syllogistically adv.
Etymology: ME f. OF silogisme or L syllogismus f. Gk sullogismos f. sullogizomai (as SYN-, logizomai to reason f. logos reason)
Useful english dictionary. 2012.