When the index falls by 7%, a so-called circuit breaker is triggered, in which trading is halted for 15 minutes.
The stoppage on Thursday morning came after a speech by President Donald Trump on Wednesday night did little to calm market worries about the economic damage from the coronavirus outbreak, which the World Health Organization is now calling a pandemic. Before stocks opened, the futures contracts for the major indices had fallen 5% to their limit-down levels, also resulting in a halt in trade.
As of Wednesday, the Dow Jones Industrial Average was down by slightly more than 20% from the record highs earlier this year, meaning the index had officially entered bear-market territory.