n 1  :  passage from one state or stage to another : esp  :  an abrupt change in energy state or level (as of an atomic nucleus or a molecule) usu. accompanied by loss or gain of a single quantum of energy  2  :  a genetic mutation in RNA or DNA that results from the substitution of one purine base for the other or of one pyrimidine base for the other .
 
  chiefly Brit   
 
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