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 loops were built in the 19th century. However they suffered from intense forces due to their circular shape. The first vertical loop was on flip flap railway even though that coaster had 12 g's so it went through the loop very quickly