A rational expression in mathematics is an expression that is the ratio of two polynomials. Most often, it is expressed as a fraction, such as (x^2 + 5)/(x - 2).
A polynomial is a mathematical expression that can have constants, variables and exponents. The various terms can be combined using addition, subtraction, multiplication and division, but certain restrictions apply. There can be no negative or fractional exponents, and it cannot contain a number divided by a variable, such as 1/x. Despite the name "polynomial," meaning "many terms," even a single term can be called a polynomial. For that reason, all polynomials can be called rational expressions, since they can all be rewritten as a fraction with 1 as the denominator.