Murder is the technical term for a form of homicide qualified by certain criteria that differ in detail (cf. Infanticide). Historically, the distinction between murder and manslaughter was made either on the basis of psychological or ethical criteria.
As the etymology shows (Old English morðor, “secret killing,” “mortal sin,” ultimately from IE *mer-, “to die”; cf. Medieval Latin mordrum, French meurtre, German Mord), the qualifying criterion for murder as opposed to “open” killing was or…