rigid designator

noun
  1. logic an expression that identifies the same individual in every possible world: for example, "Shakespeare" is a rigid designator since it is possible that Shakespeare might not have been a playwright but not that he might not have been Shakespeare
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012