A unit of radiation dose that is used for radiation protection purposes.

When an individual is exposed to mixed sources of radiation, the total biologically effective dose is calculated by multiplying the physical dose (expressed in units called gray) of each kind of radiation by a corresponding factor (called a quality factor or Q factor) specified for the type of radiation and its energy, after which these amounts are added together.

The factor for gamma rays is 1; therefore, 1 Sv = 1 Gy. The factor for the neutrons in atomic-bomb radiation is 10; therefore, 1 Sv = 0.1 Gy.

Also see Basics about radiation.