Noun
English Wikipedia has an article on:session musicianWikipedia
session musician (plural session musicians)
(music) A professional musician employed by other musicians or singers, either to play live or to make recordings
In most of the stuff that I've done over the years as a sideman, I wasn't really a session musician, because to me, a session musician is a guy who makes his living in the studio, and I never really did that. David Sanborn
What a good session musician does is listen to the song, to the artist, and to the other players. That way you can help bring out the song and help the artist express what they want to express. It's never about you stepping out and showing you can play something fancy. Benmont Tench