1. Relating to reason; not physical; mental.

2. Having reason, or the faculty of reasoning; endowed with reason or understanding; reasoning.

3. Agreeable to reason; not absurd, preposterous, extravagant, foolish, fanciful, or the like; wise; judicious; as, rational conduct; a rational man.

4. (Chem.) Expressing the type, structure, relations, and reactions of a compound; graphic; -- said of formul[ae].

Rational quantity (Alg.), one that can be expressed without the use of a radical sign, or in extract parts of unity; -- opposed to irrational or radical quantity.