What term is used to describe a roller coaster that uses only gravity for propulsion?

The correct term to describe a roller coaster that uses only gravity for propulsion is "gravity coaster." This type of roller coaster relies entirely on the natural force of gravity to move the train along the track from a higher elevation to a lower elevation. The coaster is typically designed so that it starts at a high point, and the force of gravity pulls it down, allowing it to gain speed as it descends.

In contrast, powered coasters utilize additional mechanical means, such as motors or chains, to propel the train along the track. These coasters do not depend solely on gravitational forces and can have areas where the train climbs back up, which would not be the case for a purely gravity-driven coaster.

Launch coasters use a different mechanism to achieve rapid acceleration and can propel a train from a standstill to high speeds without relying on a drop. Similarly, acceleration coasters also rely on mechanical systems to provide thrust and speed rather than gravity alone. Thus, "gravity coaster" is the most accurate term for a coaster that operates purely on gravitational energy.