motive

Definition 1

Law: Inducement, reason, or willful desire (purpose) behind the commission of an offense. Whether good (such as helping a patient suffering from unbearable chronic pain to commit suicide) or bad (such as to commit murder) it is normally not a deciding factor in determination of guilt or innocence (but 'intent' is). However, it may be relevant in a libel case, or may be invoked by a defending attorney in mitigation of punishment or by a prosecuting attorney as circumstantial evidence to prove guilt.

Definition 2

Psychology: Concern, desire, emotion, or need that energizes people to action and determines their choice of behavior.