Energy flux is the rate of transfer of energy through a surface. The quantity is defined in two different ways, depending on the context. In the first context, it is the rate of energy transfer per unit area (SI units: W·m?2 = J·m?2·s?1). This is a vector quantity, its components being determined in terms of the normal (perpendicular) direction to the surface of measurement. This is sometimes called energy flux density, to distinguish it from the second definition. Radiative flux, heat flux, and sound energy flux are specific cases of energy flux density. In the second context, it is the total rate of energy transfer (SI units: W = J·s?1). This is sometimes informally called energy current.