The Wikipedia Definition:

A game jam is a hackathon for video games. It is a gathering of people for the purpose of planning, designing, and creating one or more games within a short span of time, usually ranging between 24 and 72 hours, and some even longer. Participants are generally made up of programmers, game designers, artists, writers, and others in game development-related fields.

Traditionally, game jams focus on video games; however, board games have also been the subject of game jams