The first meaning of tan that mentions the Royal Spanish Academy (RAE) in your dictionary refers to give something the tonality of bronze: that is, the characteristic yellowish-reddish color of the alloy that combines copper and tin.
Usually, tanning refers specifically to what is done when the skin takes on a dark hue. This change in coloration can be produced by a artificial substance or through Sun rays.
The skin tans when a increase of melanin, which is a pigment present in the cytoplasm of some cells of humans and the rest of vertebrate animals. The ultraviolet light of the sunlight It favors the synthesis of melanin: in this way, if a person wants to tan, he must expose himself to solar radiation.
When tanning, an individual’s skin can acquire a color similar to bronze, although that darkening depends on the genetics each. Currently in many western countries tanning is sought for aesthetic issues.
It is important to bear in mind that the act of tanning can generate health benefits because the sun is key to the synthesis of vitamin D (which contributes to the strengthening of the bones and to achieve an improvement of the body’s defenses). However, if exposure to the sun is exaggerated, Burns on the skin and the risk of Cancer.
It should be noted, on the other hand, that there are Cosmetic products that are used to tan the skin. It is about the bronzers, composed of substances that stimulate the production or activation of melanin.