Lycium (Boxthorn) is a genus of the nightshade family (Solanaceae), containing about 90 species of plants native throughout much of the temperate and subtropical zones of the world. They are mostly found in dry, semi-saline environments.

Other common names include desert-thorn, Christmas berry, wolfberry, Matrimony vine, and Duke of Argyll's tea tree. Goji is a common English name made popular by several American-made juices and dried berries sometimes branded as "Tibetan" or "Himalayan" goji berries, although these terms do not geographically represent where the berries actually originate.

There are ~20 species in North America, ~30 species in South America, ~30 species in Africa, ~10 species in Eurasia, and one species in Australia. Grabowskia and Phrodus join Lycium in the tribe Lycieae, and are the genera most closely related to boxthorn.

The generic name is derived from the Greek word λυκιον (lycion), which was a name applied by Pliny the Elder (23-79) and Pedanius Dioscorides (ca. 40-90) to a plant known as dyer's buckthorn. It was probably a Rhamnus species and was named for Lycia, the province in which it grew.