Londinium, also known as Roman London, was the capital of Roman Britain during most of the period of Roman rule. Most twenty-first century historians think that it was originally a settlement established shortly after the Claudian invasion of Britain, on the current site of the City of London, around 47–50 AD, but some defend an older view that the city originated in a defensive enclosure constructed during the Claudian invasion in 43 AD. Its earliest securely-dated structure is a timber drain of 47 AD. It sat at a key ford at the River Thames which turned the city into a road nexus and major port (which was built between 49 and 52 AD and the city was rebuilt. It had probably largely recovered within about a decade. During the later decades of the 1st century, Londinium expanded rapidly, becoming Britannia's largest city, and it was provided with large public buildings such as a forum and amphitheatre. By the 2nd century, Londinium had grown to perhaps 30,000 or 60,000 people, almost certainly replacing Camulodunum (Colchester) as the provincial capital, and by the mid-2nd century, Londinium was at its height. Its forum basilica was one of the largest structures north of the Alps when Emperor Hadrian visited Londinium in 122. Excavations have discovered evidence of a major fire that destroyed much of the city shortly thereafter, but the city was again rebuilt. By the second half of the 2nd century, Londinium appears to have shrunk in both size and population.
Although Londinium remained important for the rest of the Roman period, no further expansion resulted. Londinium supported a smaller but stable settlement population as archaeologists have found that much of the city after this date was covered in dark earth—the by-product of urban household waste, manure, ceramic tile, and non-farm debris of settlement occupation, which accumulated relatively undisturbed for centuries. Some time between 190 and 225, the Romans built a defensive wall around the landward side of the city. The London Wall survived for another 1,600 years and broadly defined the perimeter of the old City of London.
Name
The etymology of the name Londinium is unknown. Following Geoffrey of Monmouth's pseudohistorical History of the Kings of Britain, it was long published as derived from an eponymous founder named Lud, son of Heli. There is no evidence that such a figure existed. Instead, the Latin name was probably based on a native Brittonic place name reconstructed as *Londinion. alternatively, the local pronunciation in British Latin may have changed the pronunciation of Londinium to Lundeiniu or Lundein, which would also have avoided i-mutation in Old English. The list of the 28 Cities of Britain included in the 9th-century History of the Britons precisely notes London in Old Welsh as Cair Lundem or Lundein.
The pronunciation of Londinium in English is , and its pronunciation in Classical Latin is .
Location
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The site guarded the Romans' bridgehead on the north bank of the Thames and a major road nexus shortly after the invasion. It was centred on Cornhill and the River Walbrook, but extended west to Ludgate Hill and east to Tower Hill. Just prior to the Roman conquest, the area had been contested by the Catuvellauni based to the west and the Trinovantes based to the east; it bordered the realm of the Cantiaci on the south bank of the Thames.
The Roman city ultimately covered at least the area of the City of London, whose boundaries are largely defined by its former wall. Londinium's waterfront on the Thames ran from around Ludgate Hill in the west to the present site of the Tower in the east, around . The northern wall reached Bishopsgate and Cripplegate near the former site of the Museum of London, a course now marked by the street "London Wall". Cemeteries and suburbs existed outside the city proper. A round temple has been located west of the city, although its dedication remains unclear.
Substantial suburbs existed at St Martin-in-the-Fields in Westminster and around the southern end of the Thames bridge in Southwark, where excavations in 1988 and 2021 have revealed an elaborate building with fine mosaics and frescoed walls dating from 72 AD. Inscriptions suggest a temple of Isis was located there.
Status
Londinium grew up as a vicus and soon became an important port for trade between Roman Britain and the Roman provinces on the continent. Tacitus wrote that at the time of the uprising of Boudica, "Londinium... though undistinguished by the name of 'colony', was much frequented by a number of merchants and trading vessels." Most of these were constructed near the time of the city's foundation around 47 AD. The roads are now known by Welsh or Old English names, as their original Roman names have been lost because of the lack of written and inscribed sources. (It was customary elsewhere to name roads after the emperor during whose principate they were completed, but the number and vicinity of routes completed during the time of Claudius would seem to have made this impractical in Britain's case.)
The road from the Kentish ports of Rutupiae (Richborough), Dubris (Dover), and Lemanis (Lympne) via Durovernum (Canterbury) seems to have first crossed the Thames at a natural ford near Westminster before being diverted north to the new bridge at London. which is sometimes attributed to reduced taxation. Archaeologists have found evidence that a small number of wealthy families continued to maintain a Roman lifestyle until the middle of the 5th century, inhabiting villas in the southeastern corner of the city and importing luxuries. Gildas described a revolt of Saxon foederati Bede dates it to a few years after 449 and opines that invasion had been the Saxons' intention from the beginning; the Anglo-Saxon Chronicle dates the revolt to 455. The Anglo-Saxon Chronicle records that the Britons fled to Londinium in terror after their defeat at the Battle of Crecganford (probably Crayford), its large church on Tower Hill burnt to the ground. and 60,000. possibly as the regional economic centres developed, and Londinium as the main port for imported goods became less significant. The Antonine Plague which swept the empire from 165 to 180 may have had an effect. Pottery workshops outside the city in Brockley Hill and Highgate appear to have ended production around 160.
The capital city of Londinium had inhabitants who were not natives of Britannia; they clustered in administrative and military centres, as migration was closely tied to imperial governance rather than to the permanent settlement of entire diasporas. They were from across the Roman Empire, overwhelmingly from continental Europe, particularly from Gaul and the Rhineland in Germania. Archaeological findings suggest individuals from the Middle East and North Africa were also present, but these represent rare anomalies tied to imperial governance rather than any permanent settlement. The average height for male Londoners was and the average height for female Londoners was .
Excavation
thumb|upright|A [[Romano-Celtic temple being excavated at 56 Gresham Street]]
Many ruins remain buried beneath London, although understanding them can be difficult. Owing to London's geology, which consists of a Taplow Terrace deep bed of brickearth, sand, and gravel over clay, Roman gravel roads can only be identified as such if they were repeatedly relayered or if the spans of gravel can be traced across several sites. The minimal remains from wooden structures are easy to miss, and stone buildings may leave foundations, but as with the great forum they were often dismantled for stone during the Middle Ages and early modern period. that it had been built over a Roman temple to the goddess Diana. The extensive rebuilding of London in the 20th century and following the German bombing campaign during World War II also allowed for large parts of old London to be recorded and preserved while modern updates were made. The construction of the London Coal Exchange led to the discovery of the Roman house at Billingsgate in 1848.
In the 1860s, excavations by Augustus Pitt Rivers uncovered a large number of human skulls and almost no other bones in the bed of the Walbrook. The discovery recalls a passage in Geoffrey of Monmouth's pseudohistorical History of the Kings of Britain where Asclepiodotus besieged the last remnants of the usurper Allectus's army at "Londonia". Having battered the town's walls with siegeworks constructed by allied Britons, Asclepiodotus accepted the commander's surrender only to have the Venedotians rush upon them, ritually decapitating them and throwing the heads into the river "Gallemborne". Asclepiodotus's siege was an actual event that occurred in 296, but further skull finds beneath the 3rd-century wall place at least some of the slaughter before its construction, leading most modern scholars to attribute them to Boudica's forces.
In 1947, the city's northwest fortress of the city garrison was discovered. In 1954, excavations of what was thought to have been an early church instead revealed the London Mithraeum, which was relocated to permit building over its original site. The building erected at the time has since been demolished, and the temple has been returned to its former location under the Bloomberg building.
Archaeologists began the first intensive excavation of the waterfront sites of Roman London in the 1970s. What was not found during this time has been built over, making it very difficult to study or discover anything new. Another phase of archaeological work followed the deregulation of the London Stock Exchange in 1986, which led to extensive new construction in the city's financial district. From 1991, many excavations were undertaken by the Museum of London's Archaeology Service, although it was spun off into the separately-run MOLA in 2011 following legislation to address the Rose Theatre fiasco.
Displays
thumb|A reconstructed [[Ancient Roman cuisine|Roman kitchen () at the Museum of London (2014)]]
Major finds from Roman London, including mosaics, wall fragments, and old buildings, were formerly housed in the London and Guildhall Museums. These merged after 1965 into the present Museum of London, sited from 1976 until 2022 near the Barbican Centre. The Museum of London Docklands, a separate branch dealing with the history of London's ports, opened on the Isle of Dogs in 2003. Other finds from Roman London continue to be held in the British Museum. The southwestern tower of the Roman fort northwest of town can still be seen at Noble Street. Occasionally, Roman sites are incorporated into the foundations of new buildings for future study, but these are not generally available to the public.