The painted sheet which the characters are filmed on top of. Background styles vary wildly from show to show and place to place; studios like Disney, for instance, are well-known for their sumptuous backgrounds. Even today, when studios all over the world are relying more and more heavily on computer animation techniques, backgrounds are often still hand-painted for the right effect. In terms of collectability, backgrounds on their own aren't worth very much, but adding the matching background to a cel (even a really mediocre one) can send its value soaring.