A Cosmic Void is a subset of a Universe, which doesn't contain a lot of galaxy clusters, although it contains 0.01% of galaxies
The Cosmic Void is the set of all space of a Universe, which doesn't contain any matter. It is therefore intuitive, that the union of the Cosmic Void and all matter is the Universe itself. In the case of our Universe, this "matter" would be the Cosmic Web, or Supercluster Complexes.
Sometimes, specific Cosmic Voids are labeled by name, mostly due to their proportions, but sometimes also for their contents, or lack thereof.