A condition caused by a diver decompressing too quickly. This can be caused by ascending too quickly, not carrying out necessary decompression stops, or ascending to altitude after diving.
The actual problem is with nitrogen coming out of solution in body tissues too quickly and forming bubbles - sort of like shaking a can of soda and opening it quickly. These bubbles can lead to joint pain and tiredness, and in extreme cases embolisms, strokes, and even death.
Decompression sickness can be avoided by following decompression tables and avoiding ascending to altitude after diving. Treatment is normally with recompression in a recompression chamber and oxygen therapy.