A term coined by movie critics used to describe the violent movies that were popular in Hong Kong during the 80s. Popularized by directors such as John Woo, Tsui Hark, and Ringo Lam, these movies strongly focused on themes such as honor, loyalty, brotherhood, and revenge, all set to the background of high violence (usually with guns) and a high body count. The climax would almost always be a huge battle with lots of bloodshed, hence the moniker.
Check out "A Better Tomorrow", "City on Fire", "The Killer", "Hard Boiled", "Bullet in the Head", "The Mission".