I heard a report on the radio this morning about a big football match in Kolkata between two local teams - a Kolkata derby (basic details of teams here).
Why is a match between local teams called a derby (pronounced dar-bee in British English)? Derby is a city in the north of England, but the first Derby, the horse race run annually at Epsom, was named after the Earl of Derby, a senior politician who inaugurated the race in 1780. In the 19th century the term Derby was used for important horse races elsewhere in the world, eg the French Derby. In the early 20th century the meaning of the word derby widened to mean any important sporting match, and if two local teams were involved, then derby was usually prefaced by the word local.
That is the OED's explanation of how derby came to mean a match between two local sides, but there are other theories. One of them suggests that the name is somehow linked with a 'football' match that takes place on Shrove Tuesday every year in Ashbourne - which is not Derby, but it is in the county of Derbyshire. Hundreds of people move en masse through the town all afternoon and evening playing the Ashbourne Royal Shrovetide Football match. I put football in inverted commas above, because it bears no relation to today's game with its strict rules; it is more like the original medieval game which involved unruly mobs. Most participants must never even catch a glimpse of the ball. The match has been called ‘Royal’ since 1928 when the then Prince of Wales (later Edward VIII) attended. The current Prince of Wales, Prince Charles, tossed the ball into the crowd to start the match in 2003. He, sensibly, did not take part in the game – his royal predecessor had come away with a bloody nose! You can read about this event and watch a video of a previous occasion here.