The ghost pepper, also known as bhut jolokia, is extremely hot. In the world of peppers, this hotness is measured based on the Scoville scale. The more Scoville heat units (SHU) that a pepper has, the spicier it is. For example, the bell pepper has zero SHU, while jalapeño and chipotle peppers measure 10,000 SHU each.
The ghost pepper measures a whopping 1,041,427 SHU! Individual ghost peppers may measure more or less SHU, but this is the widely-accepted average. It was the very first pepper to measure over a million SHU.