Lewis & Short

rătĭōcĭnātīvus, a, um, adj. [ratiocinor].

  1. I. In rhet., of or belonging to reasoning, syllogistic, ratiocinative: genus, Cic. Inv. 1, 13, 17: quaestio, Quint. 7, 1, 60: status, id. 7, 8, 3; cf. id. 3, 6, 46; 61; 5, 10, 6.
  2. II. In gram.: ratiocinativa conjunctio, serving for inference, illative, as ergo, igitur, Diom. p. 410 P.