Chemical imbalance is a term used, particularly but not exclusively in medicine, to describe a situation where different chemical substances required for correct functioning of a system are not present in the required proportions. Example of the use of the expression in medicine and other areas (agriculture, aquariums, etc.) are manifold [http://www.webmd.com/hw/diabetes_1_2/tp21136.asp, ^*," target="_blank" >[http://www.reasons.org/resources/connections/200411_connections_q4/index.shtml ^*.

See the article on chemical imbalance (mental health) for the idea that a chemical imbalance is the cause of mental illness.