Céilí (Irish reformed spelling), or Ceilidh (Scottish and older Gaelic spelling), pronounced Kay-Lee in either case, is the traditional Gaelic social dance in Ireland and Scotland. In the old days, before discos and nightclubs, there were céilís in most town and village halls on Friday or Saturday nights and even now they are not uncommon. Originally céilís facilitated courting and prospects of marriage for young people and, although discos and nightclubs have displaced céilís to a considerable extent, they are still an important and popular social outlet in rural parts of Ireland and Scotland, especially in the Gaelic-speaking west coast regions. Céilís are sometimes held on a smaller scale in private or public houses, for example in remote rural hinterlands and during busy festivals.

Céilí music is provided by any assortment of fiddle, flute, tin whistle, accordion, bodhrán (pronounced Bough-Rawn) which is a wooden frame covered with the stretched goat hide, and in more recent times also drums and electric bass guitar. The music is cheerful and lively and requires knowledge of basic "123 123" dance steps.

The general format of céilí dancing is the "Set". A Set consists of four couples, with each pair facing another in a square or rectangular formation. Each couple exchanges position with the facing couple, and also facing couples exchange partners, while all the time keeping in step with the beat of the music.

However, about half of the dances in the modern Scots ceilidh are couple dances performed in a ring. These can be performed by fixed couples or in the more sociable "progressive" manner, with the lady moving to the next gentleman in the ring at or near the end of each repetition of the steps.

There is also a form of dancing which in the US would be called square or line dancing - the "Siege of Ennis", "The Walls of Limerick" and "The Stack of Barley" being the most popular dances in this genre; some of the céilí dance formations are named after famous historical battles and events, others after items of daily rural life as the last three examples show.

Step dancing is another form of dancing often performed at céilís, the form that has been popularised in recent years by the world famous Riverdance ensemble. Whereas Set dancing involves all present, whatever their skill, Step dancing is usually reserved for show, being performed only by the most talented of dancers.

The céilí has been internationalised by the Scottish and Irish diasporas in Canada, the United States, Australia and New Zealand, where local céilís and traditional music competitions are held. In recent years, céilí and traditional music competitions have been frequently won by descendants of emigrants.

The equivalent of a céilí in Brittany is called a fest-noz.