NOUN (2)

1. a series of chemical reactions in which the product of one is a reactant in the next;

2. a self-sustaining nuclear reaction; a series of nuclear fissions in which neutrons released by splitting one atom leads to the splitting of others;

WordNet (r) 3.0 (2006):

chain reaction
    n 1: a series of chemical reactions in which the product of one
         is a reactant in the next
    2: a self-sustaining nuclear reaction; a series of nuclear
       fissions in which neutrons released by splitting one atom
       leads to the splitting of others