The area near the front of the stage at punk rock and metal concerts where fans will run into each other continously. This is also known as slamdancing. It is not uncommon for people to shove their friends into the pit. Normally, the pit isn't very dangerous, and most will follow the rules of a friendly pit. For example, if someone falls down in the pit, the other moshers are expected to pick them up and get them out of harms way.
There are also variatons on the mosh pit, including circle pits, and the wall of death