1. Having or exercising the ability to reason. See Synonyms at logical.
2. Consistent with or based on reason or good judgment; logical or sensible: rational decisions.
3. Of sound mind; sane: wondered if the eccentric neighbor was not completely rational.
4. Mathematics Capable of being expressed as a quotient of integers.
A rational number.
[Middle English racional, from Old French racionel, from Latin ratiōnālis, from ratiō, ratiōn-, reason; see REASON.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.