Definition: Google maintains several data centers that feed information into Google search, and these systems must be periodically updated with new Web sites.
Rather than taking all of Google off-line when the index is updated, Google takes one data center at a time off-line and updates them individually.
This is pretty standard procedure for any service that needs to be online 24 hours per day. However, because Google is so popular, occasionally people will notice a data discrepancy during this process. The same search might produce different results, depending on which data center is answering the query.
This stage of bouncing results is called the Google dance by search engine optimization experts. The Google dance usually occurs about once per month.
