adj.1 illogical; unreasonable.
2 not endowed with reason.
3 Math. (of a root etc.) not rational; not commensurate with the natural numbers (e.g. a non-terminating decimal).
Derivatives:
irrationality n. irrationalize v.tr. (also -ise). irrationally adv.
Etymology: L irrationalis (as IN-(1), RATIONAL)