The aurochs (Bos primigenius; /ˈɔːrɒks/ or /ˈaʊrɒks/; pl.: aurochs or aurochsen) is an extinct species of bovine, considered to be the wild ancestor of modern domestic cattle. With a shoulder height of up to 1.8 m (5 ft 11 in) in bulls and 1.55 m (5 ft 1 in) in cows, it was one of the largest herbivores in the Holocene; it had massive elongated and broad horns that reached 80 cm (31 in) in length.

The aurochs was part of the Pleistocene megafauna. It probably evolved in Asia and migrated west and north during warm interglacial periods. The oldest-known aurochs fossils date to the Middle Pleistocene. The species had an expansive range spanning from Western Europe and North Africa to the Indian subcontinent and East Asia. The distribution of the aurochs progressively contracted during the Holocene due to habitat loss and hunting, with the last known individual dying in the Jaktorów forest in Poland in 1627.

There is a long history of interaction between aurochs and humans, including archaic hominins like Neanderthals. The aurochs is depicted in Paleolithic cave paintings, Neolithic petroglyphs, Ancient Egyptian reliefs and Bronze Age figurines. It symbolised power, sexual potency and prowess in religions of the ancient Near East. Its horns were used in votive offerings, as trophies and drinking horns.

Two aurochs domestication events occurred during the Neolithic Revolution. One gave rise to the domestic taurine cattle (Bos taurus) in the Fertile Crescent in the Near East that was introduced to Europe via the Balkans and the coast of the Mediterranean Sea. Hybridisation between aurochs and early domestic cattle occurred during the early Holocene. Domestication of the Indian aurochs led to the zebu cattle (Bos indicus) that hybridised with early taurine cattle in the Near East about 4,000 years ago. Some modern cattle breeds exhibit features reminiscent of the aurochs, such as the dark colour and light eel stripe along the back of bulls, the lighter colour of cows, or an aurochs-like horn shape.

Both "aur" and "ur" are Germanic or Celtic words meaning "wild ox". In Old High German, this word was compounded with ohso ('ox') to ūrohso, which became the early modern Aurochs. The Latin word "urus" was used for wild ox from the Gallic Wars onwards.

The use of the plural form aurochsen in English is a direct parallel of the German plural Ochsen and recreates the same distinction by analogy as English singular ox and plural oxen, although aurochs may stand for both the singular and the plural term; both are attested.

The scientific name Bos taurus was introduced by Carl Linnaeus in 1758 for feral cattle in Poland. The scientific name Bos primigenius was proposed for the aurochs by Ludwig Heinrich Bojanus who described the skeletal differences between the aurochs and domestic cattle in 1825, published in 1827. The name Bos namadicus was used by Hugh Falconer in 1859 for cattle fossils found in Nerbudda deposits. Bos primigenius mauritanicus was coined by Philippe Thomas in 1881 who described fossils found in deposits near Oued Seguen west of Constantine, Algeria.

In 2003, the International Commission on Zoological Nomenclature placed Bos primigenius on the Official List of Specific Names in Zoology and thereby recognized the validity of this name for a wild species.