A Continuously Variable Transmission (CVT) is a form of transmission in which a gradual, smooth change in ratio from a high "gear" to a low "gear" can be achieved. This is different from a multi-speed gear system, which only allows sudden switches between a limited number of gear ratios, with the changes smoothened with the use of a clutch.
On mopeds, a CVT typically consists of two pulleys linked by a common belt. One pulley acts as the variator, which drives the change in gear ratio. The other pulley transfers power to the rest of the transmission system, and is sometimes also used to compensate for the changing belt tension caused by the varying of the variator.