A music session is a social gathering of musicians and singers who perform music in a relatively informal context. 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.

Usually traditional sessions are based around acoustic instruments. In Scotland as in Ireland, the most popular instruments include acoustic guitar, fiddle, accordion, mandolin, pipes, whistle, and sometimes double bass. Because an acoustic guitar is relatively quiet, guitarists often experience difficulty being heard over louder instruments such as accordions and whistles.

A small amplifier may be used to bring the guitar up to an audible level. However, there are a number of traditionalists who resent the use of guitar amplifiers at an otherwise acoustic session.

Depending on the session, some beginning performers may use chord charts or sheet music or no music notation at all, which is typically the case at a session with experienced participants and/or professionals. In sessions where no music notation is used, the participants or leader "call" well-known tunes that the other performers know "by ear", either by stating the title aloud, or more simply by starting into the melody.

In jazz and blues (and related genres and subgenres), a session is typically called a jam session or a jam. Jam sessions often take place after musicians and singers have finished their paid gigs for the evenings. Jams often take place at nightclubs or bars. Jams may include both professional instrumentalists and singers and talented amateurs. Some venues have a "house band" or backing band, a paid rhythm section that includes one or two chordal instrumentalists (e.g., piano, Hammond organ, electric guitar, etc.), a bassist (e.g., electric bass or double bass) and a drummer.