What is a game jam?

From the Global Game Jam website:

"Jam" is a reference to musician jam sessions, not traffic or spreadable fruit.
The goal is to come together and make a video game, or non-digital game like a board game or card game. Participants rapidly prototype game designs and hopefully inject new ideas to help grow the game industry. We share a common theme and constraints. We ask participants to create a game from beginning to end in a prescribed time.