Natural radiation, also known as background radiation, is a type of radiation that is naturally present in the environment. It is emitted from the Earth's crust, outer space, and even from within our own bodies. The term 'natural radiation' can refer to both ionizing and non-ionizing radiation. Ionizing radiation is more dangerous and includes gamma rays, X-rays, and alpha and beta particles. Non-ionizing radiation is less dangerous and includes radio waves, microwaves, and infrared radiation. The crossword solution for the term 'natural radiation' is AURA, which stands for Atmospheric Ultraviolet Radiation Assessment. This is a program that monitors the amount of ultraviolet radiation in the atmosphere.