Mass killing and related terms refer to crimes where many peoples are killed, with various definitions.
Mass murder, spree killing, serial killing, and mass shooting
"Mass murder", "spree killing", and "serial killing" may refer to different kind of killings of several persons, with the killer(s) being individuals or (small) groups. In the United States, "mass shooting" may be defined as an indiscriminate shooting of four or more people who are killed.
Regarding various racial aspects, including on mass shootings, see Race and crime: Serial and other multiple murderers.
Democide, genocide, and politicide
"Mass killing" is a general term, but may more specifically refer to very large-scale killings, often organized by countries, in this sense being a more general term than genocide. "Democide" is a similar term. There are also similar other terms, such as "politicide". They may differ regarding what groups are targeted and regarding whether the killings are intentional. Claims of such mass killings are often very politically controversial and there are very often disputes regarding their claimed existence, claimed numbers, claimed causes, and other claimed related events.
Genocides and other forms of large-scale mass killings have existed throughout human history. However, today it is overwhelmingly the argued genocide during the Holocaust that is mentioned and described in the mass media. See also the article on Holocaust uniqueness.
See the "External links" for lists and descriptions of various large-scale mass killing.