A zero-g roll, also known as as Spiraling Camelback on B&M coasters that ride above the track and Heartline Spin on coasters that ride below the track. It is a roller coaster inversion where the track twists 360 degrees (like an in-line twist or barrel roll) while cresting an airtime hill. At its highest point, riders are fully inverted.
Although the zero-g roll is designed to give the sensation of weightlessness, this isn't always the case as it depends on what speed the element is taken at.
The first roller coaster to include a zero-g roll is also the first inverted roller coaster; Batman The Ride at Six Flags Great America. Built by Bolliger & Mabillard, other companies including Vekoma, Gerstlauer and Mack Rides have since produced zero-g rolls.