The Hollywood Studio Symphony (sometimes the Hollywood Freelance Studio Symphony) is the credited name of the symphony orchestra behind many major soundtracks, including The Chronicles of Narnia: The Lion, the Witch and the Wardrobe, Sucker Punch, Jurassic Park III, Last Samurai, Pirates of the Caribbean, We Are Marshall, Spider-Man 2, Lost and The Bourne Supremacy.
Contrary to what its name might suggest, the orchestra is not a conventional symphony orchestra, in that its members are all freelance musicians who are contracted individually, and differ between individual soundtracks.
The use of the "Hollywood Studio Symphony" name is part of a 2001 contract negotiated between the American Federation of Musicians and the Alliance of Motion Picture and Television Producers to provide an incentive for soundtracks to be recorded with Los Angeles musicians instead of foreign musicians and recording venues (often in London and Prague) who were cheaper.
In the new contract, the AFM musicians agreed to lower their record rates in exchange for individual credits to all the musicians that perform the score as well as recognition for the local community venture.
Thus, the "Hollywood Studio Symphony" name is given to the entire ensemble in addition to the individual musician credits as a way to brand the LA effort.