Roll is a type of motion of an object around its longitudinal axis, so that the longitudinal axis changes direction in the horizontal plane.
Roll is a basic movement of an aircraft, ship, or projectile. The basic aerodynamic or hydrodynamic means of changing roll are opposed movements of the ailerons, or combined structures such as flaperons or elevons.
Spacecraft make changes in roll with reaction motors at located around the longitudinal axis, with their exhausts directed at a tangent to the longitudinal axis. Certain highly maneuverable aircraft also use reaction controls for quick changes in roll.