The Dow Jones industrial average spooked Wall Street on Tuesday by forming a chart pattern that's supposed to signal the end of bull markets ... the so-called death cross.
Investopedia defines the theory as:
A crossover resulting from a security's long-term moving average breaking above its short-term moving average or support level.
The common way to measure this is comparing the 50-day moving average to the 200-day moving average. When the 50-day falls below the 200-day, a "death cross" is formed and a securities long-term trend could be over.