Normally, in an eight-person formation "Centers" means four dancers. However, in some cases it is useful to be able to refer to just the two dancers who are closest to the center of the overall formation.
The terms used, interchangeably, for this are the "Very Centers" and the "Center 2".
In formations like these, the rest of the dancers (the ones who are not the Center 2) are typically referred to as the "Outside 6".