See rational in Webster's New World College Dictionary
adjective
of, based on, or derived from reasoning: rational powers
able to reason; reasoning; in possession of one's reason or sanity
showing reason; not foolish or silly; sensible: a rational argument
Math.
designating or of a real number or quantity expressible as the ratio of two integers, with the second integer not being equal to zero: all integers and fractions are rational numbers
designating a function expressible as the quotient of two polynomials