Monitor lizards are lizards in the genus Varanus, the only extant genus in the family Varanidae. They are native to Africa, Asia, and Oceania, and one species is also found in the southern United States as an invasive species. About 80 species are recognized.

Monitor lizards have long necks, powerful tails and claws, and well-developed limbs. The adult length of extant species ranges from 20 cm (7.9 in) in some species such as Varanus sparnus, to over 3 m (10 ft) in the case of the Komodo dragon, though the extinct megalania (Varanus priscus) may have reached lengths of more than 7 m (23 ft). Most monitor species are terrestrial, but many are also arboreal or semiaquatic. While most monitor lizards are carnivorous, eating smaller reptiles, fish, birds, insects, small mammals, and eggs, a few species also eat fruit and vegetation.

The generic name Varanus is derived from the Arabic word ورل waral (Standard Arabic, colloquially ورر [warar] or ورن [waran]), from a common Semitic root ouran, waran, warar or waral, meaning "lizard beast".

In English, they are known as "monitors" or "monitor lizards". The earlier term "monitory lizard" became rare by about 1920. The name may have been suggested by the occasional habit of varanids to stand on their two hind legs and to appear to "monitor", or perhaps from their supposed habit of "warning people of the approach of venomous animals". But all of these explanations for the name "monitor" postdate Linnaeus giving the scientific name Lacerta monitor to the Nile monitor in 1758, which may have been based on a mistaken idea by Linnaeus that the German word Waran (borrowed from Arabic) was connected to warnen (to warn), leading him to incorrectly Latinize it as monitor ('warner' or 'adviser').

Austronesian languages spoken across Southeast Asia, where varanids are common, have a large number of slightly related local names for them. They are usually known as biawak (Malay, including Indonesian standard variety), bayawak (Filipino), binjawak or minjawak or nyambik (Javanese), or variations thereof. Other names include hokai (Solomon Islands); bwo, puo, or soa (Maluku); halo (Cebu); galuf or kaluf (Micronesia and the Caroline Islands); batua or butaan (Luzon); alu (Bali); hora or ghora (Komodo group of islands); phut (Burmese); and guibang (Manobo).

In South Asia, they are known as hangkok in Meitei, mwpou in Boro, ghorpad (घोरपड) in Marathi, uḍumbu (உடும்பு) in Tamil and udumbu (ഉടുമ്പ്) in Malayalam, bilgoh in Bhojpuri, gohi (गोहि) in Maithili, in Sinhala as තලගොයා / කබරගොයා (talagoya [land monitor] / kabaragoya [water monitor, where 'kabara' means 'vitiligo']), in Telugu as uḍumu (ఉడుము), in Kannada as uḍa (ಉಡ), in Punjabi and Magahi as goh (गोह), in Assamese as gui xaap, in Odia as godhi (ଗୋଧି), and in Bengali as (goshaap) (গোসাপ) or (guishaap) (গুইসাপ), as goh (गोह) in Hindi, and godhā (गोधा) in Sanskrit.

The West African Nile monitor is known by several names in Yoruba, including awọ́nríwọ́n, awọ̀n, and àlégbà. In Serer and amongst the Wolofized Serers of Kaolack, it is known as mbossé (or mboose in Serer) or bar in Wolof (pronounced barrr with a long "R"). Mbossé is the tutelary and traditional totem of the city of Kaolack, (formerly part of the Serer precolonial Kingdom of Saloum, now part of modern-day Senegal). The mbossé is also one of the totems of the noble Joof family born of Guelwar matrilineage, which is one of the many Serer maternal clans. The mbossé or mbosseh (in Gambian English) is one of the Serer religious festivals, and should not be confused with the mythical and sacred tree mbos―enshrined in Serer religion and Serer cosmogony―where the mbossé (the lizard) takes its name.

Due to confusion with the large New World lizards of the family Iguanidae, the lizards became known as "goannas" in Australia. Similarly, in South African English, they are referred to as leguaans, or likkewaans, from the Dutch term for the Iguanidae, leguanen.