The SI unit of Absorbed Radiation dose (e.g. x-rays) is the Gray (symbol: Gy) named after British physicist Louis Harold Gray.
It replaces the Centimetre-Gram-Second (cgs) unit, the Rad, and is defined as:
The absorption of one joule of ionizing radiation by one kilogram of matter (usually human tissue)
More Radiation aborptions units are shown below...