The BBC offers commentary on the question, but the BBC itself was ensnared by the inherent difficulties of the topic. The Telegraph also offers a good summary.
How often I have heard or participated in an argument when someone dismissively said "It's just semantics"! I once took an undergraduate class, ENGL 3008, General Semantics. The textbook was Language in Thought and Action by S.I. Hayakawa. You may remember him from California politics; he served in the U.S. Senate. His material dealt mainly with traps of language, traps of thought, and how they interact and reinforce. Hayakawa would say definitions do matter greatly, but one doesn't have to study General Semantics to know that. Ask any marketing professional. A word or phrase can have enormous differential meaning and powerful consequences when used.
The United Nations adopted its Convention on the Prevention and Punishment of the Crime of Genocide in 1948. Almost 150 nations have ratified it or acceded to it — including Turkey. It defines genocide.
Any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such:(a) Killing members of the group;
(b) Causing serious bodily or mental harm to members of the group;
(c) Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part;
(d) Imposing measures intended to prevent births within the group;
(e) Forcibly transferring children of the group to another group.
This is a legal definition, meant for application in court. Does the Armenian atrocity meet this definition? I confess that I don't have a sufficient grasp of the facts to say conclusively that it does. I know of no attempt to prosecute under the Convention. Perhaps there has never been a prosecution, and if so the language has never been interpreted by judges and juries. But does the Armenian atrocity meet a common-sense definition of genocide? To my eyes, the weight of the evidence is that it does. But mere weight of the evidence is a shaky criterion in questions like this. The Holocaust was genocide beyond a reasonable doubt. Do we have consensus beyond a reasonable doubt that the Armenian atrocity meets a common-sense definition of genocide? It appears not yet — but the jury is out, and history has time to render a decisive verdict.