Main Entry: sub·sti·tu·tion

Pronunciation: \ˌsəb-stə-ˈt(y)ü-shən\

Function: noun

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 ; especially : 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 substitution neurosis> c : the reacting to each of a set of stimuli by a response prescribed in a key <a substitution test for speed of learning new responses>