Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometers. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered one of the additive primary colors. On the HSV color wheel, also known as the RGB color wheel, the complement of green is magenta; that is, a purple color corresponding to an equal mixture of red and blue light. On a color wheel based on traditional color theory (RYB), the complementary color to green is considered to be red.

The word green is closely related to the Old English verb growan, "to grow". It is used to describe plants or the ocean. Sometimes it can also describe someone who is inexperienced, jealous, or sick. In the United States of America, green is a slang term for money, among other things. Several colloquialisms have derived from these meanings, such as "green around the gills", a phrase used to describe a person who looks ill.