"Assassination is the murder of a prominent person or political figure by a surprise attack, usually for payment or political reasons.
An assassination may be prompted by religious, ideological, political, or military motives; it may be carried out for the prospect of financial gain, to avenge a grievance, from the desire to acquire fame or notoriety (that is, a psychological need to garner personal public recognition), from the wish to form some kind of "relationship" with a public figure, or from the desire (or at least the willingness) to be killed or commit suicide in the act."
So I guess it depends on who and why (victim and motive). The person has to be a public or political figure. Basically I guess all assassinations are technically murders, but not all murders are assassinations. Premeditated murder of someone important = assassination.