Limit down is a decrease in the price of a futures contract or stock, sufficient to trigger trading restrictions.
Restrictions can be in the form of trading stops from five minutes to the rest of the session. They can also allow you to trade at prices not lower than the limit down.
The Limit Up-Limit Down rule attempts to mitigate sudden fluctuations in the price of individual stocks.
Market-wide circuit breakers are driven by a significant intraday decline in the S&P 500.