Motive is a term that turns up both in the popular psychology of literature and cinema, and as term of art in law.
In the literary sense, motive describes the situations that fictional characters find themselves in, that impel them to move and act the way they do. Motive is particularly important to the devotees of method acting, in which the actors seek to identify psychologically with the characters they portray in order to make the characters' emotional state their own.
In law, especially criminal law, motive describes the cause that moves people and induce a certain action. Motive in itself is seldom an element of any given crime; however, the legal system typically allows motive to be proven in order to make plausible the accused's reasons for committing a crime, at least when those motives may be obscure or hard to identify with.
The law technically distinguishes between motive and intent. "Intent" in criminal law is synonymous with mens rea, which means no more than the specific mental purpose to perform a deed that is forbidden by a criminal statute, or the reckless disregard of whether the law will be violated. "Motive" describes instead the reasons in the accused's background and station in life that are supposed to have induced the crime.
Motive is particularly important in prosecutions for homicide. First, murder is so drastic a crime that most people recoil from the thought of being able to do it; proof of motive explains why the accused did so desperate an act.
Moreover, most common law jurisdictions have statutes that provide for degrees of homicide, based in part on the accused's mental state. The lesser offence of voluntary manslaughter, for example, traditionally required that the accused knowingly and voluntarily kill the victim (as in murder); in addition, it must be shown that the killing took place in the "sudden heat of passion," an excess of rage or anger coming from a contemporary provocation, which clouded the accused mind. Homicides motivated by such factors are a lesser offense than murder "in cold blood."