Often and thoughtlessly repeated, 'One man's terrorist in another man's freedom fighter' is one of those sayings that cry out for logical and philosophical analysis. Competent analysis will show that clear-thinking persons ought to avoid the saying.
Note first that while freedom is an end, terror is a means. So to call a combatant a terrorist is to say something about his tactics, his means for achieving his ends, while to call a combatant a freedom fighter is to say nothing about his tactics or means for achieving his ends. It follows that one and the same combatant can be both a terrorist and a freedom fighter. For one and the same person can employ terror as his means while having freedom as his end.
Another Palestinian renounces terrorism and fights for freedom from occupation by the path of negotiation. He is objectively a freedom fighter and objectively no terrorist. A third case might be an Israeli terrorist who blows up a Palestinian hospital or mosque in revenge for Palestinian terrorist attacks. He is objectively a terrorist but objectively not a freedom fighter.
So there are two reasons to avoid 'One man's terrorist is another man's freedom fighter.' The first is that it rests on a confusion of means and ends. Describing a combatant as a terrorist, I describe his means not his end; describing a combatant as a freedom fighter, I describe his end not his means. A second reason to avoid the saying is because the saying suggests falsely that there is no fact of the matter as to whether or not a person is a terrorist. There is: a combatant is a terrorist if and only if he employs terror as a tactic in the furtherance of his political goals. It doesn't matter what his goal or end is. It might be the noble one of freedom from oppression. Or it might be base one of domination and exploitation. What makes him a terrorist is the means he employs.
In brief, terror is a means not an end, and there is an objective fact of the matter whether a combatant is a terrorist or not. But what is a terrorist? I suggest that the following are all essential marks of a terrorist. I claim they are all individually necessary conditions for a combatant's being a terrorist; whether they are jointly sufficient I leave undecided. 'Terrorist' is used by different people in different ways. That is not my concern. My concern is how we ought to use the term if we intend to think clearly about the phenomenon of terrorism and keep it distinct from other phenomena in the vicinity.
1. A terrorist aims at a political objective. Criminals may 'terrorize' as when a loanshark microwaves a delinquent's cat, but criminals who terrorize are not terrorists because their aim is personal, not political. And although terrorists commit crimes, they are best not classified as criminals for the same reason. Treating the 1993 and 2001 attacks on the World Trade Center as criminal matters shows a lack of understanding of the nature of terrorism.
2. A terrorist does not discriminate between combatants and noncombatants. All are fair game, which is not to say that in a particular situation a terrorist might not have a reason not to target some combatants or some noncombatants. This distinguishes a terrorist organization such as Hezbollah from the Israeli Defense Forces. As a matter of policy, the IDF does not target noncombatants, whereas as a matter of policy Hezbollah targets anyone on the enemy side. The deliberate targeting of civilians also distinguishes terrorists from guerilla fighters.
3. A terrorist is not an agent of a state but of a nonstate or substate entity. A terrorist is neither a criminal (see #1 above) nor a warrior; a terrorist act is neither a criminal act nor an act of war; a terrorist organization is neither a criminal gang nor a state. Strictly speaking, only states make war.
Of course, a state (e.g. Iran) can arm and support and make use of a terrorist outfit (e.g. Hezbollah) in pursuit of a political objective (e.g., the destruction of Israel). But that does not elide the distinction between states and terrorist organizations. It is also clear that states sometimes 'terrorize'; but this is not a good reason to think of states as terrorist organizations, or some or all of their combatants as terrorists or of any of their acts as terrorist acts. The Allied firebombing of Dresden in February of 1945 was a deliberate targeting of combatants and noncombatants alike in clear violation of 'just war' doctrine. But whatever one's moral judgment of the Dresden attack or the nuclear attacks on Hiroshima and Nagasaki, none of these acts count as terrorist for the simple reason that they were the acts of states, not terrorist organizations. Some will bristle at this, but if one wants to think clearly about terrorism one must not confuse it with other things.
4. A terrorist is not a saboteur. Sabotage is one thing, terrorism another. Analytical clariy demands a distinction. Infecting computer networks with malware or attacking the power grid are acts of sabotage, but they are not strictly speaking acts of terrorism. An act is not terrorist unless it involves the killing or maiming of human beings or the threat thereof.
I am indebted to the discussion in Louise Richardson, What Terrorists Want, Random House, 2006, Ch. 1