Colosseum is an oval amphitheatre in the center of the city of Rome, Italy, just east of the Roman Forum.
The site chosen was a flat area on the floor of a low valley between the Caelian, Esquiline and Palatine Hills, through which a canalised stream ran as well as an artificial lake/marsh. By the 2nd century BC the area was densely inhabited. It was devastated by the Great Fire of Rome in 64 AD, following which Nero seized much of the area to add to his personal domain. He built the grandiose Domus Aurea on the site, in front of which he created an artificial lake surrounded by pavilions, gardens and porticoes. The existing Aqua Claudia aqueduct was extended to supply water to the area and the gigantic bronze Colossus of Nero was set up nearby at the entrance to the Domus Aurea. Originally, the building's Latin name was simply the Latin: amphitheatrum, lit. 'amphitheatre'. Though the modern name Flavian Amphitheatre (Latin: Amphitheatrum Flavium) is often used, there is no evidence it was used in Classical Antiquity. This name refers to the patronage of the Flavian dynasty, during whose reigns the building was constructed, but the structure is better known as the Colosseum.In antiquity, Romans may have referred to the Colosseum by the unofficial name Amphitheatrum Caesareum (with Caesareum an adjective pertaining to the title Caesar), but this name may have been strictly poetic as it was not exclusive to the Colosseum; Vespasian and Titus, builders of the Colosseum, also constructed a Flavian Amphitheatre in Puteoli (modern Pozzuoli).

The name Colosseum is believed to be derived from a colossal statue of Nero on the model of the Colossus of Rhodes. The giant bronze sculpture of Nero as a solar deity was moved to its position beside the amphitheatre by the emperor Hadrian (r. 117–138).The word colosseum is a neuter Latin noun formed from the adjective colosseus, meaning "gigantic" or "colossean".By the year 1000 the Latin name "Colosseum" had been coined to refer to the amphitheatre from the nearby "Colossus Solis".

The spelling was sometimes altered in Medieval Latin: coloseum and coliseum are attested from the 12th and 14th centuries respectively.In the 12th century, the structure was recorded as the amphitheatrum colisei, 'Amphitheatre of the Colossus'.In the High Middle Ages, the Flavian amphitheatre is attested as the late 13th-century Old French: colosé, and in Middle French as: colisée by the early 16th century, by which time the word could be applied to any amphitheatre.From Middle French: colisée derived the Middle English: colisee, in use by the middle of the 15th century and employed by John Capgrave in his Solace of Pilgrims, in which he remarked: Middle English: collise eke is a meruelous place … þe moost part of it stant at þis day. An English translation by John Bourchier, 2nd Baron Berners, of Antonio de Guevara's biography of Marcus Aurelius (r. 161–180) in about 1533 referred to Middle English: this Emperour, beynge with the Senate at Collisee ....Similarly, the Italian: colosseo, or coliseo, are attested as referring first to the amphitheatre in Rome, and then to any amphitheatre (as Italian: culiseo in 1367).By 1460, an equivalent existed in Catalan: coliseu; by 1495 had appeared the Spanish: coliseo, and by 1548 the Portuguese: coliseu.