n 1  :  a chemical reaction in which one or more atoms or groups in a molecule are replaced by equivalent atoms or groups to form at least two products : esp  :  the replacement of hydrogen in an organic compound by another element or group  2 a  :  the turning from an obstructed desire to another desire whose gratification is socially acceptable  b  :  the turning from an obstructed form of behavior to a different and often more primitive expression of the same tendency <a neurosis>   c  :  the reacting to each of a set of stimuli by a response prescribed in a key <a test for speed of learning new responses>  .
 
 
 
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