"Our class enemies are in the habit of complaining about our terrorism. What they mean by this is rather unclear. They would like to label all the activities of the proletariat directed against the class enemy's interests as terrorism. The strike, in their eyes, is the principal method of terrorism. The threat of a strike, the organisation of strike pickets, an economic boycott of a slave-driving boss, a moral boycott of a traitor from our own ranks - all this and much more they call terrorism. If terrorism is understood in this way as any action inspiring fear in, or doing harm to, the enemy, then of course the entire class struggle is nothing but terrorism. And the only question remaining is whether the bourgeois politicians have the right to pour out their flood of moral indignation about proletarian terrorism when their entire state apparatus with its laws, police and army is nothing but an apparatus for capitalist terror!
However, it must be said that when they reproach us with terrorism, they are trying - although not always consciously - to give the word a narrower, less indirect meaning. The damaging of machines by workers, for example, is terrorism in this strict sense of the word. The killing of an employer, a threat to set fire to a factory or a death threat to its owner, an assassination attempt, with revolver in hand, against a government minister - all these are terrorist acts in the full and authentic sense. However, anyone who has an idea of the true nature of international Social Democracy ought to know that it has always opposed this kind of terrorism and does so in the most irreconcilable way.
Why?"
Leon Trotsky's 1909 - Why Marxists oppose Individual Terrorism

We do not need UN to define terrorism. We need UN to defeat terrorism.