the ability of a substance to flow easily. Fluidity definition is - the quality or state of being fluid. the quality or state of being fluid. Fluidity, generally, refers to the tendency of any substance to distort its shape in order to flow or to take the shape of the container it is contained in at normal. …the viscosity is called the fluidity, a measure of the ease of flow. Look up fluidity in Wiktionary, the free dictionary.