A limit down price is the maximum sell-off permitted in a market on a single day of trading. Once this level has been reached, trading on the market may then be restricted to prevent significant volatility and potential panic selling. A limit down price is typically determined as a percentage decline in a given market, rather than a nominal decline in price.
A limit down period is imposed by an exchange (such as the NYSE) and not by brokers. It usually lasts 15 minutes but may be extended depending on the percentage decline before the market open.
Please note that a limit down only restricts selling on the affected market(s).