Tan lines are division between an area or areas on the skin of a human of pronounced comparative paleness relative to other areas of their body that have been suntanned by prolonged exposure to ultraviolet radiation. The source of the radiation may be the sun, or artificial UV sources as used in tanning beds. The tanned area is such that it becomes visually clear where the body of the person was exposed versus the place(s) they covered up. We're loving this series of tanned skin from the art duo Kozyndan.
Tan-line areas can occur more generally in almost any context where the human body is exposed to comparatively more sunlight in certain areas than in others. In some cases very little sunlight is required: people can sometimes pale even further than the natural skin tone of the rest of their body. In women, paling further than natural skin tone sometimes occurs in the areas of the breasts typically covered by the brassiere or a bathing suit top. Paling can also occur due to coverage around the buttocks or groin area where clothes are especially taut. Tan lines are usually an inadvertent result of a work environment or of recreational activities, but are sometimes a personal choice.