The word pavilion entered English in the 13th century from Old French pavillon, which meant both a large tent and, remarkably, a butterfly. The Old French word descends from Latin papilionem, the accusative form of papilio, which in classical Latin meant butterfly. The semantic leap from butterfly to tent occurred because Roman soldiers and camp followers compared the spread flaps of a large field tent to the outstretched wings of a butterfly. This metaphor proved so durable that it has persisted through two thousand years of linguistic change, even as the connection between butterflies and buildings has become invisible to modern speakers.
The Latin word papilio is of uncertain deeper origin. Some etymologists have proposed that it involves a reduplication of a root meaning to flutter, which would make the word onomatopoeic or imitative in nature. No consensus has been reached on a specific Proto-Indo-European source, and papilio may be a pre-Latin substrate word. What is certain is that papilio originally and primarily meant butterfly in classical Latin, and the tent meaning developed as a secondary, metaphorical extension.
The journey through Old French transformed both the word's form and its semantic range. The 12th-century Old French pavillon could mean a large tent, a canopy, or a banner, and these senses carried into Middle English when the word was borrowed in the 13th century. The butterfly meaning was left behind in Latin and early Romance; French retained it separately as papillon, which remains the standard French word for butterfly today.
The cognates across Romance languages clearly show the Latin source. French papillon (butterfly) and pavillon (pavilion, tent) represent the split between the two Latin meanings. Italian padiglione means pavilion; Spanish pabellon means pavilion and also flag or banner. All descend from the same Latin papilionem, each language preserving slightly different aspects of the original semantic range.
The PIE root *pleth2-, meaning flat or broad, has been proposed as a distant connection to papilio through the concept of flatness or spreading, but this is speculative. What is well established is that the related English words place (via French from Latin platea), plaza (via Spanish from Latin platea), and piazza (via Italian from Latin platea) all derive from a Greek word meaning broad, connecting the architectural sense of pavilion to a broader semantic field of open, flat spaces.
In English, pavilion has undergone steady semantic expansion since the 13th century. Its original meaning of a large ornamental tent persisted through the 17th century and is still used in historical and literary contexts. By the 18th century, pavilion had extended to mean a light, often open-sided structure used for shelter or entertainment, the garden pavilion or bandstand pavilion familiar in parks. In cricket, the pavilion is the clubhouse where players and spectators gather, a usage dating to the 19th century. In modern commercial usage, pavilion can refer to a large exhibition hall, a wing of a hospital, or a section
The word's extraordinary journey from a Latin butterfly through a Roman military tent metaphor to a modern sports clubhouse or exhibition hall demonstrates how metaphor can permanently alter a word's meaning. No English speaker encountering the word pavilion today would connect it to butterflies, yet the connection is etymologically direct and historically documented.