On a stock chart, the Death cross occurs when the 50-day MA falls below the 200-day MA. As the name implies, a Death Cross is associated with sharp downward price movement and can be used as a sell signal in the belief that a significant downtrend will follow.
The reverse of this event is known as a Golden Cross where the 50-day MA rises above the 200-day MA, a bullish signal.
We discuss the interpretation of its opposite number, the Golden Cross in more depth here.