means the quality of thinking, deciding, or acting based on logic, reason, and evidence rather than emotion or impulse.

from From Latin rationalis, 'endowed with reason,' built on ratio — a wonderfully overworked word meaning 'reckoning, calculation, reason, account.' That ratio comes from reri, 'to reckon or think,' and is the same root that gives us ratio, ration, and rate — all of them about measuring and dividing things out. English took 'rational' in the late Middle Ages via Latin and Old French, with the abstract '-ity' noun trailing along behind, so that the word for clear-eyed thought is, fittingly, rooted in arithmetic.