In many European- and South American-based professional sports leagues, a mechanism by which teams change divisions (levels of play) between seasons.

In its simplest form, a promotion/relegation system promotes the top team or teams in each division below the top (Premiership, Serie A, etc.) to the division immediately above it, and relegates the last-place team or lowest teams in each division above the bottom to the division below in turn. Most versions are a bit more complex, generally involving a playoff for the privilege of promotion or to avoid the disgrace of relegation.

One major implication of this idea is that teams in any division who have aspirations of being competitive (which should be all teams) must be logistically capable of handling promotion, in terms of stadium size, fan base, income, etc. As I explored in my minor league baseball WU, this is one reason why promotion/relegation systems have never been seriously explored in any North American professional sport -- for example, our largest Class AAA baseball stadia seat maybe 22,000; the smallest major league baseball stadium, Fenway Park in Boston, seats 33,817, but is a special historic case -- 40,000 is a bare minimum these days. Professional football and basketball don't even have viable minor leagues whose seasons run concurrently with those of the NFL and NBA; hockey struggles enough to fill seats as it stands.