|  | 
Definition of Gy
1. Noun. The SI unit of energy absorbed from ionizing radiation; equal to the absorption of one joule of radiation energy by one kilogram of matter; one gray equals 100 rad. 
Medical Definition of Gy
1. 
|  | 
1. Noun. The SI unit of energy absorbed from ionizing radiation; equal to the absorption of one joule of radiation energy by one kilogram of matter; one gray equals 100 rad. 
1.