ultraviolet index; UV index
A measure of the strength of ultraviolet radiation from the sun, commonly used in weather forecasts and health advisories.
Today's UV index is very high, so please apply sunscreen.
紫外線 refers to ultraviolet radiation itself, while 紫外線指数 is the numerical index measuring its intensity.
Compound of 紫外線 (shigaisen, 'ultraviolet rays') and 指数 (shisū, 'index').