What is Virtual Incumbent?

Legal Definition
The virtual incumbent or quasi-incumbent is the candidate in an election who campaigns as though he or she currently holds the office being contested, though the actual incumbent is not running for re-election. Traditionally, the quasi-incumbent will be the nominee from the party of the sitting office-holder. In the 2008 U.S. presidential election, however, virtual incumbency was also determined less formally, either by the policies of the actual candidates or the state of the polls.
-- Wikipedia