Definition
US markets halt trading market-wide when the S&P 500 falls 7%, 13% or 20% from the prior close; the first two levels pause trading for 15 minutes during most of the day, and the third ends trading for the day. Individual stocks also have limit up-limit down bands that pause a stock when it moves too far, too fast.
Watch out for
Halts are meant to let information catch up, not to stop declines.