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Definition of Gray
Medically reviewed by Min Clinic Staff | Updated: January 2026
Gray: A unit of absorbed radiation equal to the dose of one joule of energy absorbed per kilogram of matter, or 100 rad. The unit is named for the British physician L. Harold Gray (1905-1965), an authority on the use of radiation in the treatment of cancer. The abbreviation for a gray is Gy. See also: Milligray.
Gray is, of course, also a hue between black and white.