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background radiation

Pronunciation:   bak-grownd rA-dE-A-shun
Definition : Background radiation comes from natural sources and is always present in the environment. This includes solar and cosmic radiation as well as radioactive elements in the ground, building materials, and the human body.
Facts: Average annual dose of background radiation for an American is about 360 millirems.

References: Definition courtesy of Frontline: Nuclear Reaction.