Vis a tergo is a Latin term that refers to an external force that propels or drives an entity forward. There are several synonyms for vis a tergo, including impulsion, impetus, propulsion, momentum, and thrust. These words depict the idea of an external force that provides the energy to move something forward. Impulsion and impetus imply a sudden or strong force driving something forward or into action. Propulsion refers specifically to the force that propels an object forward, such as the thrust from an engine. Momentum refers to the quantity of motion that an object possesses, and thrust refers to the force that pushes something forward or upward.