Something that maintains a controlled state.
[Greek statikos, causing to stand.]
Most terms refer to an instrument, such as thermostat, one that automatically regulates temperature; rheostat (Greek rheos, stream), one that controls a current by varying the resistance; humidistat, a device that regulates the humidity of the air in a room or building; cryostat (Greek kruos, frost), one that maintains a very low temperature; haemostat (US hemostat)(Greek haima, blood), an instrument for preventing blood flow by compression of a blood vessel.
Others refer to a substance that controls a system, or a system so controlled, such as chemostat, a system in which the chemical composition is kept at a controlled level, especially for the culture of micro-organisms, or bacteriostat, a substance that stops bacteria multiplying without destroying them. The appestat is the region of the hypothalamus of the brain which is believed to control a person's appetite for food.