HOW DOES THE UN DEFINE TERRORISM? It seems fairly straight forward. The U.S. government defines it at 18 USC 2331 as

activities that involve violent acts . . . dangerous to human life that are a violation of the criminal laws of the [US], . . . and that appear to be intended . . . to intimidate or coerce a civilian population, . . . to influence the policy of a government by intimidation or coercion; or to affect the conduct of a government by mass destruction, assassination, or kidnapping . . . .

The U.S. came up with this definition in the wake of 9-11.

As to the UN, incredibly, they still have not defined terrorism. As Eye on the UN explains:

On Thursday February 15, 2007 the UN concluded its eleventh session of the Ad Hoc Committee set up to negotiate a draft comprehensive convention on international terrorism. It ended by agreeing to meet again - 8 months from now.
Rather than fulfilling the promise of September 2005's "Reform" Summit of world
leaders to define terrorism and finalize this treaty, stalemate was the result of two weeks of negotiations and the free spending of your taxpayer dollars. Why the impasse? Cutting through the verbiage, there are many states at the UN that think it's acceptable to kill civilians - especially Jewish and American ones - and there are many other states that refuse to stand up to the thugs.

Read the whole story.