The sievert with the symbol Sv is the unit of measurement for various weighted doses of radiation. The sievert is used as the unit for the effective dose and equivalent dose.
The unit was named after the Swedish medical physicist Rolf Sievert. Earlier, the unit used for the effective dose was the rem (1 rem = 0.01 Sv).
As 1 Sv represents a relatively high effective dose, values which occur in practice are usually given in millisieverts (1 mSv = 0.001 Sv).
For the weighting, the absorbed dose is multiplied with a radiation weighting factor. As this factor is a dimensionless number, the effective dose has the same dimension as the absorbed dose, specifically the energy per mass unit transferred from the radiation to the material, i.e. J/kg. To highlight the difference between the dose sizes for practical use, this unit is known by the name Gray (Gy) for the absorbed dose, while it is known by the name Sievert for the effective dose.