Webster's English Dictionary

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rational

1. ra.tio.nal \'rash-n*l, -*n-*l\ \-e-\ aj [ME racional, fr. L rationalis, 
   fr. ration-, ratio] 1a: having reason or understanding  1b: relating to, 
   based on, or agreeable to reason  2: relating to or resulting from the 
   application of the arithmetic operati ons to integers or to polynomials 
   {division is a ~ operation} - ra.tio.nal.ly av
2. rational n : something rational; specif : a rational number or fraction