Rational may be:

• 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".
  

de:Rational fr:Rationnel