The gray (symbol: Gy) is a derived unit of ionizing radiation dose in the International System of Units (SI). It is defined as the absorption of one joule of radiation energy per one kilogram of matter.
It is used as a measure of absorbed dose, specific energy (imparted), and kerma (an acronym for “kinetic energy released per unit mass”). It is a physical quantity, and does not take into account any biological context. Unlike the pre-1971 non-SI roentgen unit of radiation exposure, the gray when used for absorbed dose is defined independently of any target material. However, when measuring kerma the reference target material must be defined explicitly, usually as dry air at standard temperature and pressure.
The equivalent cgs unit, the rad (equivalent to 0.01 Gy), remains common in the United States, though “strongly discouraged” in the style guide for U.S. National Institute of Standards and Technology authors.