Surround Processor
A surround processor is essentially an A/V receiver without the amplification stage. These are generally reserved for the very best of Home Theatre systems, and will allow you to be selective with the power amplification that you use.
The pre-amplification and processing section of an Audio Visual receiver is very sensitive, and can be subject to interference generated through the power amp stages. By isolating this and housing it in its own chassis (along with its own power supply) performance can be markedly improved.