rational |ˈra sh ənl; ˈra sh nəl| adjective 1 based on or in accordance with reason or logic : I'm sure there's a perfectly rational explanation. • (of a person) able to think clearly, sensibly, and logically : Andrea's upset—she's not being very rational. See note at sensible . • endowed with the capacity to reason : man is a rational being. 2 Mathematics (of a number, quantity, or expression) e…