Pronunciation: (rash'u-nl, rash'nl),[key] —adj. 1. agreeable to reason; reasonable; sensible: a rational plan for economic development. 2. having or exercising reason, sound judgment, or good sense: a calm and rational negotiator. 3. being in or characterized by full possession of one's reason; sane; lucid: The patient appeared perfectly rational. 4. endowed with the faculty of reason: rational beings. 5. of, pertaining to, or constituting reasoning powers: the rational faculty. 6. proceeding or derived from reason or based on reasoning: a rational explanation. 7. Math. a. capable of being expressed exactly by a ratio of two integers.
b. (of a function) capable of being expressed exactly by a ratio of two polynomials.
8. Class. Pros.capable of measurement in terms of the metrical unit or mora.