A divergence is an early warning of a potential trend change.
As long as an indicator corresponds with the price trend, the data is said to confirm the price trend. But when an indicator fails to confirm the trend, it is called a divergence. A bullish or positive divergence happens at bottoms, while a bearish or negative divergence happens at tops.
This technical concept is mostly used in combination with oscillators, but it may also happen when two or more averages or indices fail to show confirming trends. Note that divergences may also occur more than one time before a price reversal.