I'm not going to try to give a complete definition of massacre here, but at least one component.

A massacre must involve the killing of unarmed unresisting civilians, or disarmed unresisting surrendered soldiers, or mass killings in areas unable to or not contributing to a war effort. (Note one does have to include the provision "unresisting" because many/most soldiers and at least some civilians are trained in hand-to-hand combat and would pose a threat even if totally disarmed.)

An example of a possible component: a police officer shooting an unarmed criminal in the back. Another example of a probable component: throwing a grenade through a window when there is little reason to believe occupants of the room behind the window are dangerous.

As an aside, I would argue that the bombings of Hiroshima and Nagasaki were arguably massacres, because by that time Japanese industry (even if Hiroshima and Nagasaki were producing as much munitions as they could) had been so reduced in productive capability that their military significance had been greatly reduced. (By this argument, one can also argue that the fire-bombing of Tokyo itself was not a massacre although it did inflict massive civilian casualties; Tokyo at the time was very much contributing to their industrial war production and its industries could not have been effectively selectively targeted even if the technological capability had existed at the time. But I grant you that this can be argued either way.)

An example of a probable non-component: the Cincinnati riots last year were started when a police officer shot an unarmed black teenager who was apparently making motions consistent with pulling a gun out of his pants. In another incident, an apparently crazed man came after police with a brick and subsequently died after they shot him. Another example of a non-component: a civilian throwing rocks or otherwise threatening the health and livelihood of a policemen or soldier, who shoots back. (One can argue that the officer or soldier *could* have used rubber bullets, beanbag guns, or other methods to disperse the threatening civilian, but such is based on decisions made on the spot and hard to second-guess.)

What numbers you might put on it, I cannot venture to guess. In partial violation of my first sentence, I would argue that one such death is murder, rather than a massacre. I won't try to argue at this time where "murder" ends and "massacre" begins.