Genocide is understood by most to be the gravest crime against humanity it is possible to commit.
It is the mass extermination of a whole group of people, an attempt to wipe them out of existence.
But at the heart of this simple idea is a complicated tangle of legal definitions.
So what is genocide and when can that term be applied?
The term was coined in 1943 by the Jewish-Polish lawyer Raphael Lemkin who combined the Greek word "genos" (race or tribe) with the Latin word "cide" (to kill).
After witnessing the horrors of the Holocaust, in which every member of his family except his brother was killed, Dr Lemkin campaigned to have genocide recognised as a crime under international law.
His efforts gave way to the adoption of the UN Convention on Genocide in December 1948, which came into effect in January 1951.
Article Two of the convention defines genocide as "any of the following acts committed with the intent to destroy, in whole or in part, a national, ethnic, racial or religious group, as such":
- Killing members of the group
- Causing serious bodily or mental harm to members of the group
- Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part
- Imposing measures intended to prevent births within the group
- Forcibly transferring children of the group to another group
The convention also imposes a general duty on states that are signatories to "prevent and to punish" genocide.
Since its adoption, the UN treaty has come under fire from different sides, mostly by people frustrated with the difficulty of applying it to specific cases.
They argue that the definition is too narrow. Others say the term is devalued by misuse.
Some analysts contend that the definition is so narrow that none of the mass killings perpetrated since the treaty's adoption would fall under it.
The objections most frequently raised against the treaty include:
- The convention excludes targeted political and social groups
- The definition is limited to direct acts against people, and excludes acts against the environment which sustains them or their cultural distinctiveness
- Proving intention beyond reasonable doubt is extremely difficult
- UN member states are hesitant to single out other members or intervene, as was the case in Rwanda
- There is no body of international law to clarify the parameters of the convention (though this is changing as UN war crimes tribunals issue indictments)
- The difficulty of defining or measuring "in part", and establishing how many deaths equal genocide
But in spite of these criticisms, there are many who say genocide is recognisable.
In his book Rwanda and Genocide in the 20th Century, former secretary-general of Medecins Sans Frontieres, Alain Destexhe, says: "Genocide is distinguishable from all other crimes by the motivation behind it.
"Genocide is a crime on a different scale to all other crimes against humanity and implies an intention to completely exterminate the chosen group.
"Genocide is therefore both the gravest and greatest of the crimes against humanity."
Loss of meaning
But Mr Destexhe believes the word genocide has fallen victim to "a sort of verbal inflation, in much the same way as happened with the word fascist".
He says the term has progressively lost its initial meaning and is becoming "dangerously commonplace".
Michael Ignatieff, director of the Carr Centre for Human Rights Policy at Harvard University, agrees.
"Those who should use the word genocide never let it slip their mouths. Those who unfortunately do use it, banalise it into a validation of every kind of victimhood," he said in a lecture about Raphael Lemkin.
"Slavery, for example, is called genocide when - whatever it was, and it was an infamy - it was a system to exploit, rather than to exterminate the living."
The differences over how genocide should be defined have also led to disagreements on how many genocides actually occurred during the 20th Century.
History of genocide
Some say there was only one genocide in the last century: the Holocaust.
However, others say there have been at least three genocides under the 1948 UN convention:
- The mass killing of Armenians by Ottoman Turks between 1915-1920, an accusation that the Turks deny
- The Holocaust, during which more than six million Jews were killed
- Rwanda, where an estimated 800,000 Tutsis and moderate Hutus died in the 1994 genocide
In Bosnia, the 1995 massacre at Srebrenica has been ruled to be genocide by the International Criminal Tribunal for the former Yugoslavia (ICTY).
And others give a long list of what they consider cases of genocide, including the Soviet man-made famine of Ukraine (1932-33), the Indonesian invasion of East Timor (1975), and the Khmer Rouge killings in Cambodia in the 1970s.
The International Criminal Court in 2010 issued an arrest warrant for the President of Sudan, Omar al-Bashir, on genocide charges.
He is accused of waging a campaign against the citizens of the Sudanese region of Darfur.
Some 300,000 people are said to have died and millions have been displaced in seven years of fighting there.
More recently, in March 2016, the US said so-called Islamic State (IS) was carrying out genocide against Christian, Yazidi and Shia minorities in Iraq and Syria.
IS, or Daesh, was "genocidal by self-proclamation, by ideology and by actions, in what it says, what it believes and what it does," Secretary of State John Kerry said.
The jihadist group seized large swathes of territory in Syria and Iraq in 2014, and has since become notorious for its brutality against perceived opponents.
The first case to put into practice the convention on genocide was that of Jean Paul Akayesu, the Hutu mayor of the Rwandan town of Taba at the time of the killings.
In a landmark ruling, a special international tribunal convicted him of genocide and crimes against humanity on 2 September 1998.
More than 30 ringleaders of the Rwandan genocide have now been convicted by the International Criminal Tribunal for Rwanda.
In August 2010 a leaked UN report reportedly alleges that Rwandan Hutus, perpetrators of the 1994 genocide, may themselves have been victims of the same crime.
In 2004, the ICTY widened the definition of what constitutes genocide.
General Radislav Krstic, the first man convicted by the ICTY of genocide in Bosnia, had appealed against his conviction for his role in the killing of more than 7,000 Muslim men and boys in Srebrenica.
But the court rejected his argument that the numbers were "too insignificant" to be genocide - a decision likely to set an international legal precedent.
Since then a Bosnian Serb military commander has been cleared of being involved in Srebrenica.
It remains to be seen whether cases still pending will aid clarity on what is and what is not genocide.
The Bosnian Serb leader Radovan Karadzic is currently on trial at The Hague accused of war crimes and genocide. He denies the charges.
President Bashir continues to travel outside Sudan, to countries who are signatories to the International Criminal Court, without being detained as ordered by the arrest warrant.
If his case is ever brought to trial it will be the first time that genocide charges are brought against a serving head of state.