- In chemistry, replacement of an atom or group in a compound by another atom or group (substitution of H by Cl in CH4 to give CH3Cl).
- In psychoanalysis, an unconscious defense mechanism by which an unacceptable or unattainable goal, object, or emotion is replaced by one that is more acceptable or attainable; the process is more acute and direct, and less subtle, than sublimation.
[L. substitutio, to put in place of another]
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Examples: glitazone, GI cocktail, etc.