/
ˌratɪɒsɪˈneɪʃ(ə)n, 
ˌraʃɪ-/
n.
 formal the forming of judgements by a process of logic; reasoning.
Derivative
- ratiocinate v.
  - ratiocinative adj.
  - ratiocinator n.
 
Etymology
C16: from L. ratiocinatio(n-), from ratiocinari ‘deliberate, calculate’, from ratio (see ratio).