[ir.ra.tio.nal] adj [ME, fr. L irrationalis, fr. in- + rationalis rational] (14c): not rational: as a (1): not endowed with reason or understanding (2): lacking usual or normal mental clarity or coherence b: not governed by or according to reason "~ fears" c Greek & Latin prosody (1) of a syllable: having a quantity other than that required by the meter (2) of a foot: containing such a syllable d (1): being an irrational number "an ~ root of an equation" (2): having a numerical value that is an irrational number "a length that is ~" -- ir.ra.tio.nal.i.ty n -- ir.ra.tio.nal.ly adv
[2]irrational n (1646) 1: an irrational being
2: irrational number