late 14c., "endowed with reason," from L. rationalis "of or belonging to reason, reasonable," from ratio (gen. rationis) "reckoning, calculation, reason" (see ratio). Rationalist "physician whose treatment is based on reason" is from 1620s; applied to a philosophical doctrine
1640s. Rationalize is first recorded 1803, "to explain, to make reasonable;" in the psychological sense of "to give an explanation that conceals true motives" it dates from 1922.
[Mathematics] a fractional number n/d, where n and d are integers, n is the numerator and d is the denominator. The set of all rational numbers is usually called Q. Computers do not usually deal with rational numbers but instead convert them to real numbers which are represented (approximately in some cases) as floating-point numbers. Compare irrational.