If I may be fully deconstructionist for a moment...
The 'definition' of a word is a matter of conventional usage. In other words, we develop a loose sense (a template) of how a word is used in various discursive contexts, and fit it to specific instances in order to convey meaning. Usually this process is non-problematic. Nouns like (say) box, star, or rabbit don't often produce confusion, and when they do — because the edges of definitions are fuzzy and gray, with things that might or might not be boxes, stars, or rabbits — we fudge through with gestures and clarifications. A definition is never meant to be 'exact'; it's meant to convey a sense of things so that others can grasp our meaning.
Think of that old psychological saw that what you see as red may not be anything like what I see as red, but the word 'red' helps us talk about (say) different kinds of apples effectively.
However, the usage of some terms is intrinsically non-systematic, so that the fuzzy, gray edges wash out any conventional consensus on a definition. 'Terrorism' falls in that category. I mean, we all loosely understand that 'terrorism' is defined as inflicting or threatening harm against 'innocent' (civilian or non-combatant) populations with the goal of forcing concessions from a political elite. It's a kind of hostage-taking, where the lives and welfare of an entire civilian population are threatened by the terrorists in demand for concessions from state institutions that (ostensibly) are responsible for those citizens. But in usage, the word always runs up against the fact that political leaders generally reserve the right to threaten or inflict harm against 'innocents' (be it their own citizens or foreign nationals) for the leaders' own political interests. How often do we hear about a local or national government rolling out tanks, armored vehicles, and military hardware to disperse civilian protests? The goal there is to inflict fear on the civilian population in order to preserve the political status quo, something which neatly falls under the definition of terrorism. But state actors tend to view such behavior merely as maintaining law and order.
This is why the UN definition of terrorism cagily begins with a reference to 'criminal acts': "Criminal acts intended or calculated to provoke a state of terror in the general public..." States define what a 'crime' is, and so states are largely immune to any accusations of terrorism.
So the actual 'in practice' usage of the term 'terrorism' is always schizoid, boiling down to something like:
Terrorism is the illegitimate use or threat of harm against a
civilian population for political ends, where legitimacy is determined
by sovereign state actors within a largely anarchic international
realm.
Because the definition of terrorism always hinges on idiosyncratic, state-determined concepts of 'legitimacy', it's almost impossible to use in any meaningful, universal sense.
