Synchronisation fees are royalties that are paid for the use of music as part of another work, for example as background music in a film or an ad.
Synchronisation fees are one of many cash flow streams that are generated by music companies, and one that seems likely to grow (especially compared to retail sales of music).
Like mechanical royalties, synchronisation fees are usually collected by bodies that act for all major music publishers, giving them an effective monopoly on the collection of such royalties. In the UK this is the Mechanical Copyright Protection Society.