A jam session is a gathering of musicians who get together to play around with music. Improvised music is commonly played and/or sung at jam sessions
People have been playing music together for a very long time, and it's safe to say that the jam session has been around in some form or another for thousands of years. In the 1930s, however, jam sessions reached a new height in the jazz community, with musicians playing recognized standards in front of an audience, and then getting together for a heated and experimental after hours jam session. After hours sessions often included cutting contests, in which competitors cut into each others' solos in an attempt to outdo their performances.