A death cross is a bearish technical trading indicator that occurs when the 50-day moving average falls below the 200-day moving average, indicating a big sell-off.
A death cross is formed when a slower moving average crosses the faster moving average in the upward direction. The most popular moving average used by day traders is the 50-day moving average and the 200-day moving average. The slower-moving average has to cross the faster-moving average from below for a death cross to be formed on the trading charts. Other examples of death crosses can be seen in 5-day and 15-day averages, however, longer periods are more reliable and provide stronger signals of an asset/stock/cryptocurrency.