Uropygi is an arachnid order comprising invertebrates commonly known as whip scorpions or vinegaroons (also spelled vinegarroons and vinegarones). They are often called uropygids. The name "whip scorpion" refers to their resemblance to true scorpions and possession of a whiplike tail, and "vinegaroon" refers to their ability when attacked to discharge an offensive, vinegar-smelling liquid, which contains acetic acid. The order may also be called Thelyphonida. Both names, Uropygi and Thelyphonida, may be used either in a narrow sense for the order of whip scorpions, or in a broad sense which includes the order Schizomida.

Carl Linnaeus first described a whip scorpion in 1758, although he did not distinguish it from what are now regarded as different kinds of arachnid, calling it Phalangium caudatum. Phalangium is now used as a name for a genus of harvestmen (Opiliones). In 1802, Pierre André Latreille was the first to use a genus name solely for whip scorpions, namely Thelyphonus. Latreille later explained the name as meaning "qui tue", meaning "who kills". One name for the order, Thelyphonida, is based on Latreille's genus name. It was first used, as the French Thélyphone, by Latreille in 1804, and later by Octavius Pickard-Cambridge in 1872 (with the spelling Thelyphonidea).

The alternative name, Uropygi, was first used by Tamerlan Thorell in 1883. It means "tail rump", from Ancient Greek οὐροπύγιον (ouropygion), from οὐρά (oura) "tail" and πυγή (pygē) "rump" referring to the whip-like flagellum on the end of the pygidium, a small plate made up of the last three segments of the abdominal exoskeleton.

The classification and scientific name used for whip scorpions varies. Originally, Amblypygi (whip spiders), Uropygi and Schizomida (short-tailed whipscorpions) formed a single order of arachnids, Pedipalpi. Pedipalpi was later divided into two orders, Amblypygi and Uropygi (or Uropygida). Schizomida was then split off from Uropygi into a separate order. The remainder has either continued to be called by the same name, Uropygi, possibly distinguished as Uropygi sensu stricto, or called Thelyphonida. When the name Uropygi is used for the whip scorpions, the clade containing Uropygi and Schizomida may be called Thelyphonida, or Thelyphonida s.l. Conversely, when the name Thelyphonida is used for the whip scorpions alone, the parent clade may be called Uropygi, or Uropygi sensu lato. The table below summarizes the two usages. When the qualifications s.l. and s.s. are omitted, the names Uropygi and Thelyphonida are ambiguous.

Phylogenetic studies show the three groups, Amblypygi, Uropygi s.s. and Schizomida, to be closely related. The Uropygi s.s. and Schizomida likely diverged in the late Carboniferous, somewhere in the tropics of Pangaea.

Whip scorpions range from 25 to 85 mm (1.0 to 3.3 in) in length, with most species having a body no longer than 30 mm (1.2 in); the largest species, of the genus Mastigoproctus, can reach 85 mm (3.3 in). An extinct Mesoproctus from the Lower Cretaceous Crato Formation could be the same size. Because of their legs, claws, and "whip", though, they can appear much larger, and the heaviest specimen weighed was 12.4 grams (0.44 oz).

The opisthosoma consists of 12 segments. The first segment forms a pedicel, and each of the next eight segments has dorsal tergites. The last three segments are fused into closed rings, forming a pygidium that ends with a flagellum of 30–40 units.

Like the related orders Schizomida and Amblypygi, whip scorpions use only six legs for walking, with the first two legs serving as antennae-like sensory organs. All species also have very large scorpion-like pedipalps (pincers) but there is an additional large spine on each palpal tibia. They have one pair of median eyes at the front of the cephalothorax and up till five pairs of lateral eyes on each side of the head, a pattern also found in scorpions. Whip scorpions have no venom glands, but they have glands near the rear of their abdomen that can spray a combination of acetic acid and caprylic acid when they are bothered. The acetic acid gives this spray a vinegar-like smell, giving rise to the common name vinegaroon.