An aeolipile (or aeolipyle, or eolipile), also known as a Hero's engine, is a simple, bladeless radial steam turbine which spins when the central water container is heated. Torque is produced by steam jets exiting the turbine, much like a tip jet[1] or rocket engine.[2] In the 1st century AD, Hero of Alexandria described the device in Roman Egypt, and many sources give him the credit for its invention.[3][4]

The aeolipile which Hero described is considered to be the first recorded steam engine or reaction steam turbine.[5] The name – derived from the Greek word Αἴολος and Latin word pila – translates to "the ball of Aeolus", Aeolus being the Greek god of the air and wind.

Predating Hero's writings, a device called an aeolipile was described in the 1st century BC by Vitruvius in his treatise De architectura; however, it is unclear if it is the same device or a predecessor, as he does not mention rotating parts.[6]