A stadium (pl.: stadiums or stadia) is a place or venue for (mostly) outdoor sports, concerts, or other events and consists of a field or stage completely or partially surrounded by a tiered structure designed to allow spectators to stand or sit and view the event. Pausanias noted that for about half a century the only event at the ancient Greek Olympic festival was the race that comprised one length of the stadion at Olympia, where the word "stadium" originated. Most of the stadiums with a capacity of at least 10,000 are used for association football. Other popular stadium sports include gridiron football, baseball, cricket, the various codes of rugby, field lacrosse, bandy, and bullfighting. Many large sports venues are also used for concerts.

"Stadium" is the Latin form of the Greek word "stadion" (στάδιον), a measure of length equalling the length of 600 human feet. As feet are of variable length the exact length of a stadion depends on the exact length adopted for 1 foot at a given place and time. Although in modern terms 1 stadion = 600 ft (180 m), in a given historical context it may actually signify a length up to 15% larger or smaller. The equivalent Roman measure, the stadium, had a similar length – about 185 m (607 ft) – but instead of being defined in feet was defined using the Roman standard passus to be a distance of 125 passūs (double-paces). The English use of stadium comes from the tiered infrastructure surrounding a Roman track of such length. Most dictionaries provide for both stadiums and stadia as valid English plurals.

The oldest known stadium is the Stadium at Olympia in Greece, where the ancient Olympic Games were held from 776 BC. Initially the Games consisted of a single event, a sprint along the length of the stadium. Greek and Roman stadiums have been found in numerous ancient cities, perhaps the most famous being the Stadium of Domitian, in Rome. The excavated and refurbished ancient Panathenaic Stadium hosted attempted revivals of the Olympic Games in 1870 and 1875 before hosting the first modern Olympics in 1896, the 1906 Intercalated Games, and some events of the 2004 Summer Olympics. The excavation and refurbishment of the stadium was part of the legacy of the Greek national benefactor Evangelos Zappas, and it was the first ancient stadium to be used in modern times.

Stadiums in ancient Greece and Rome were built for different purposes, and at first only the Greeks built structures called "stadium"; Romans built structures called "circus". Greek stadia were for foot races, whereas the Roman circus was for horse races. Both had similar shapes and bowl-like areas around them for spectators. The Greeks also developed the theatre, with its seating arrangements foreshadowing those of modern stadiums. The Romans copied the theatre, then expanded it to accommodate larger crowds and more elaborate settings. The Romans also developed the double-sized round theatre called amphitheatre, seating crowds in the tens of thousands for gladiatorial combats and beast shows. The Greek stadium and theatre and the Roman circus and amphitheatre are all ancestral to the modern stadium.

The first stadiums to be built in the modern era were basic facilities, designed for the single purpose of fitting as many spectators in as possible. With tremendous growth in the popularity of organised sport in the late Victorian era, especially association football in the United Kingdom and baseball in the United States, the first such structures were built. One such early stadium was the Lansdowne Road Stadium, the brainchild of Henry Dunlop, who organised the first All Ireland Athletics Championships. Banned from locating sporting events at Trinity College, Dunlop built the stadium in 1872. "I laid down a cinder running path of a quarter-mile, laid down the present Lansdowne Tennis Club ground with my own theodolite, started a Lansdowne archery club, a Lansdowne cricket club, and last, but not least, the Lansdowne Rugby Football Club – colours red, black and yellow." Some 300 cartloads of soil from a trench beneath the railway were used to raise the ground, allowing Dunlop to use his engineering expertise to create a pitch envied around Ireland.

Other early stadiums from this period in the UK include the Stamford Bridge stadium (opened in 1877 for the London Athletic Club) and Anfield stadium (1884 as a venue for Everton F.C.).

In the U.S., many professional baseball teams built large stadiums mainly out of wood, with the first such venue being the South End Grounds in Boston, opened in 1871 for the team then known as the Boston Beaneaters (now the Atlanta Braves). Many of these parks caught fire, and those that did not proved inadequate for a growing game. All of the 19th-century wooden parks were replaced, some after a few years, and none survive today.

Goodison Park was the first purpose-built association football stadium in the world. Walton-based building firm Kelly brothers were instructed to erect two uncovered stands that could each accommodate 4,000 spectators. A third covered stand accommodating 3,000 spectators was also requested. Everton officials were impressed with the builder's workmanship and agreed two further contracts: exterior hoardings were constructed at a cost of £150 and 12 turnstiles were installed at a cost of £7 each. The stadium was officially opened on 24 August 1892 by Lord Kinnaird and Frederick Wall of the Football Association. No football was played; instead the 12,000 crowd watched a short track and field event followed by music and a fireworks display. Upon its completion the stadium was the first joint purpose-built football stadium in the world.