a cluster of gears consisting of a central gearwheel with external teeth (the sun), a coaxial gearwheel of greater diameter with internal teeth (the annulus), and one or more planetary gears engaging with both of them to provide a large gear ratio in a compact space
epicyclic train in American English
a system of gears, belt pulleys, etc., in which at least one gear or pulley axis moves around the circumference of another fixed or moving axis, permitting an unusually high or low velocity ratio with relative simplicity of parts