Diminished Capacity Definition

A reduced capacity, caused by mental illness, intoxication, or some other cause, that prevented a person accused of an illegal act from formulating the full intent necessary for the most serious offense with which he or she is charged.
American Heritage Medicine
An alteration to a defendant’s mental state, a reduced ability to understand, usually the result of mental retardation, alcohol or drug intoxication, or some other factor, which exists at the time of the commission of a crime, not sufficient to support an insanity defense, but that raises the issue of whether the defendant was able to form the intent to commit the crime. Typically offered as a defense in partial mitigation to obtain conviction on a lesser included charge or to receive a lesser sentence.
Webster's New World Law