Mirroring is when one site hosts an identical (or near identical) copy of what's hosted on a different site. This can happen for a variety of reasons: e.g. the content is hosted in a politically unstable location and other sites will mirror the content in case the main site goes offline or becomes inaccessible. Mirroring is common amongst wikis, especially Wikipedia. Sometimes, a site will only host a portion of the original material, usually related to a specific concept.
When the mirrored material is substantially changed and starts to take on a different life of its on, this is known as forking and results in increasingly separate communities and resources.
See also federated wiki (concept).