Rational

pertaining to rationality
acting according to the philosophical principles of rationalism
a mathematical term for certain numbers; the rational numbers
A rational function, a mathematical function which can be written as the ratio of two polynomial functions.
the software company Rational Software; now owned by IBM, and formerly Rational Software Corporation
the most central paradigm of Rational-Emotive Therapy
Rational, one of the four temperaments according to the Keirsey Temperament Sorter.
A common misspelling of "rationale".