A restrictor is a device used to limit or control the flow or movement of something. There are various synonyms for the word restrictor, including limiter, governor, controller, regulator, and constrainer. A limiter or governor is a device used to limit the speed or power of a machine or vehicle. A controller is a device used to regulate the amount of energy or power supplied to a system. A regulator is a device used to control or maintain the pressure, temperature, or flow of a fluid or gas in a system. A constrainer is a device used to restrict or limit the movement or activity of a person or animal.