A music session is a social gathering of musicians and singers who perform music in a relatively informal context.^[1] Much of the music performed at such events is traditional music for the area, popular songs and other well-known tunes. In sessions, the participants typically improvise the accompaniment, song arrangements and musical ornaments to the melodies of songs or tunes. The venue may be a public bar, tavern, village hall or a private home.

The events usually occur at regular intervals and a weekday evening may be chosen if professional musicians are expected to take part, so that it does not interfere with paid weekend gigs. More amateur players may organize a session on a weekend. At a session, musicians usually play or sing for pleasure and to socialize. Frequently the musicians and singers are unpaid. However, one or two musicians (usually experienced players or professionals) are paid to 'lead' a session and to encourage others to join in. In a jazz or blues jam, the venue may pay a small "house band" (typically a rhythm section) to provide accompaniment.

Depending on the genre and region, music sessions may be called a "traditional music session", "session", or in jazz and blues, a "jam session" or "jam".

The music session forms an important part of Scottish and Irish culture (see Irish traditional music session). Often such events form an integral part of music festivals. The festival will often consist of a series of concerts by local groups, well-known or famous bands or entertainers. It is quite common for the local amateurs and touring professional musicians to meet after the concerts and have a "session" late in the evening. These sessions may continue into early hours of the morning. In jazz and blues, "jam sessions" are an important part of the musical culture.

Paid session leaders help to ensure that there will be music performances throughout the evening for members of the public who might wish to listen without participating. The leader also acts as a host and master of ceremonies for the event, introducing performers and, thanking them after their songs and calling for applause. The session leader selects players and singers who are skilled enough to deserve a solo performance, and politely asks inexperienced performers to wait for later in the session.

If a session performer is exceptionally skilled, the leader may encourage her to perform a number of songs or tunes. On the other hand, if a player or singer does not perform that well, due to a lack of experience or technique, the leader will thank the individual for his or her performance and then diplomatically ask for the audience's applause, while at the same time inviting a more seasoned performer on stage to take the first performer's place. Less skilled performers may be asked to participate as a member of an ensemble, playing an accompaniment role in the background.

Some session leaders use a sign-up sheet where participants can indicate their interest in performing. The session leader is often an accomplished singer and/or instrumentalist, and she typically performs throughout the evening, either in solo roles or "comping" for (accompanying) other performers. The session leader's ability to perform helps to fill in any periods during the evening where no participants have asked to come up.

The performers and singers who participate in an unpaid session typically do so for the enjoyment of performing and for the applause they receive from the audience. In jazz, jam sessions are often a good way for players and singers to meet other performers; in a sense, it can serve as a form of business networking for musicians. At a jam session, jazz musicians can hear other performers on stage, meet with them, and consider certain individuals for collaboration on future paid gigs or recording sessions. A jazz singer, for example, might use attendance and participation in jam sessions as a way to find potential members who she could hire as a rhythm section for her shows. A bandleader might use participation in jam sessions as a way to scout out new talent for her big band. In jazz, jam sessions are also an opportunity for informal learning, as players can show other performers new playing techniques or improvisation approaches.

The hospitality of the landlord or venue owner can also be important to the music session scene. Sometimes he or she might offer a round of drinks or tea, coffee and food to the performers, to thank them for their contribution to the entertainment of the audience.

As part of jazz festivals, the organizers sometimes arrange what are termed "jam sessions", which are concerts in which only professional musicians perform. While these shows may share the informal feel of regular jam sessions, they are different from the truly informal jam sessions that are held, at which both professionals and talented amateurs share the stage, and at which none of the solo performers (except the host/MC) are being paid.