Relative permittivity is a term commonly used in physics and electrical engineering to describe the ratio of the permittivity of a substance to that of a vacuum. However, this term can also be described using several other synonyms. For instance, some people use "dielectric constant" to mean the same thing as relative permittivity. Similarly, "electric susceptibility" or "permittivity constant" can be used interchangeably with relative permittivity. Other words that can be used synonymously with relative permittivity include capacitance ratio, electrical constant, and electric permittivity. Regardless of which term is used, they all refer to the same physical property that dictates the ability of a material to store electrical energy in an electric field.