High Quality Content by WIKIPEDIA articles! A plateau of a function is a part of its domain where the function has constant value.In mathematics, the domain (sometimes called the source) of a given function is the set of "input" values for which the function is defined. For instance, the domain of cosine would be all real numbers, while the domain of the square root would be only numbers greater than or equal to 0 (ignoring complex numbers in both cases). In a representation of a function in a xy Cartesian coordinate system, the domain is represented on the x axis (or abscissa).