A jam session is a musical event, process, or activity where musicians play (i.e. "jam") by improvising without extensive preparation or predefined arrangements. Jam sessions are often used by musicians to develop new material (music), find suitable arrangements, or simply as a social gathering and communal practice session. Jam sessions may be based upon existing songs or forms, may be loosely based on an agreed chord progression or chart suggested by one participant, or may be wholly improvisational. Jam sessions can range from very loose gatherings of amateurs to evenings where a jam session coordinator acts as a "gatekeeper" to ensure that only appropriate-level performers take the stage, to sophisticated improvised recording sessions by professionals which are intended to be edited and released to the public.

Jam session is musicians' informal group playing.