A vertical loop is a roller coaster element in which the track makes a 360 degree oval shape vertically, as opposed to a horizontal loop which is essentially a helix. Inverting riders once, it is the first known inversion and the most common today.
A relatively new element is the non-inverted loop which is shaped like a vertical loop, however the track continually twists in such a way that riders are never inverted.
The first vertical loop was on the Flip Flap Railway, which was built in the 19th century. However, the lack of upstop wheels meant that the train needed to go through the loop at a very high speed in order to stay on the track. This, combined with the loop's circular shape, meant that riders were exposed to 12 g's during the loop, which resulted in injuries.
The first modern vertical loop was built on the Revolution at Six Flags Magic Mountain in 1976. This loop has a clothoid or upside-down teardrop shape rather than a circular shape, which helps to reduce forces. In 2000, Son of Beast opened at Kings Island, which was the first modern wooden coaster with a loop. However, this coaster got very rough, and was eventually removed in 2009 and replaced by Banshee.