/ ˌrætiˌɒsɪˈneɪʃn; NAmE ˌreɪʃioʊ-/ noun
[ U ] ( formal ) the process of thinking or arguing about sth in a logical way
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WORD ORIGIN
mid 17th cent.: from Latin ratiocinat- deliberated, calculated, from the verb ratiocinari , from ratio (see ratio ).