Murder is the direct and voluntary killing of an innocent human being.
Murder is direct in the moral sense of the word, not necessarily physically direct. If you devise a clever physically-indirect way to kill an innocent human being, such as by some type of Rube-Goldberg device, it is still morally direct. In a morally direct act of murder, the chosen act is inherently ordered toward a certain end, the death of an innocent human being. This end is called the moral object; it is the end toward which the chosen act is intrinsically directed, by the very nature of the act.
Murder is voluntary in the sense of deliberate, i.e. intentional. The act that is done is chosen by the human will and intellect; it is a knowingly chosen act. The act is chosen knowing that the act is directed toward the killing of an innocent human person.
The definition of murder is entirely in the second font of morality, which is the chosen act with its inherent moral meaning (i.e. its essential moral nature or moral 'species' -- the type of act in terms of morality). This inherent moral meaning of a knowingly chosen act is determined by its moral object. In the case of murder, the chosen act is intrinsically evil because it is intrinsically directed at the moral object of killing an innocent human person.
Abortion is murder in the circumstance where the human person is a prenatal (unborn). Infanticide is murder in the circumstance where the human person is an infant (born). The circumstances (third font) makes one act of murder abortion and another act of murder infanticide. But the essential moral nature of the act, murder, is entirely in the second font, as determined by the moral object.
Pope John Paul II: "Therefore, by the authority which Christ conferred upon Peter and his Successors, in communion with the Bishops--who on various occasions have condemned abortion and who in the aforementioned consultation, albeit dispersed throughout the world, have shown unanimous agreement concerning this doctrine--I declare that direct abortion, that is, abortion willed as an end or as a means, always constitutes a grave moral disorder, since it is the deliberate killing of an innocent human being. This doctrine is based upon the natural law and upon the written Word of God, is transmitted by the Church's Tradition and taught by the ordinary and universal Magisterium." (Evangelium Vitae, n. 62.)
Roman Catholic theologian