A reel is type of tune in 2/4 or 4/4 time, for which there is a dance type also called the reel, common in "Celtic" musics such as Scottish traditional music and Irish traditional music. Reels are reputed to have originated in Scotland, and many (but far from all) reels in currency in Irish traditional music can be traced back to Scottish originals. Both countries have played reels for well over a century, however, and there has been a fair bit of borrowing back from Ireland to Scotland. The term "reel" may be most familiar to some in the name the "Virginia reel"; but that is not a type of tune but a type of dance.
Reels are also played in England, Canada, and other places with Scottish and Irish immigration. In the United States, outside of immigrant traditions, reels are played especially in contradance music. In American old time music from Appalachia and the South, tunes similar to reels are played, but are generally called breakdowns and hoedowns.