"Circuit breaker" in the financial world is an emergency brake on stock market trading, a rule instituted by an exchange or regulator that temporarily halts trading to dampen panic reactions.

For example, if the price of stocks falls by a certain percentage (e.g., 15%) within a trading day, trading may be halted for a certain period of time, such as half an hour (also called a freeze). This mechanism is intended to calm the markets and prevent panic selling from further destabilizing the market.

Circuit breakers are similar to the "emergency stop" or "emergency brake" and are used to control unexpected price declines and restore stability.