Radiation dosimetry is the measurement and calculation of the absorbed dose in matter and tissue resulting from the exposure to indirect and direct ionizing radiation. It is a scientific subspecialty in the fields of health physics and medical physics that is focused on the calculation of internal (Internal dosimetry) and external doses from ionizing radiation.
Dose is reported in gray (Gy) for matter or sieverts (Sv) for biological tissue, where 1 Gy or 1 Sv is equal to 1 joule per kilogram. Non-SI units are still prevalent as well, where dose is often reported in rads and dose equivalent in rems. By definition, 1 Gy = 100 rad and 1 Sv = 100 rem.
In the United States, radiation dosimeters measure and display dose in rems rather than sieverts. One rem = 1 centisievert = 1/100 sievert.
Read more about Dosimetry: Radiation Effects On Living Tissue, Dose Versus Activity, Measuring Dose, Medical Dosimetry, Radiation Exposure Monitoring