The Ground Axiom
  The ground axiom is the assertion that the set-theoretic universe is not obtainable by forcing over any inner model. Although this appears at first to be a second-order assertion, it is actually first-order expressible in the language of set theory. This article is the extended abstract for a talk at the 2005 Workshop in Set Theory at the Mathematisches Forschungsinstitut Oberwolfach. 
Joel David Hamkins
math.LO