The Collaborative International Dictionary of English v.0.48:

nuclear family \nuclear family\ n.
   A family consisting of parents and their children and
   grandparents of a marital partner.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

nuclear family
    n 1: a family consisting of parents and their children and
         grandparents of a marital partner [syn: nuclear family,
         conjugal family]