Strange, I could not find the definition of terrorist in The Merriam-Webster Dictionary. I found the definition of terrorism. Terrorism is defined as "The systematic use of terror to coerce by threat or violence." By this definition, terrorism is a behavior. By default, we can define a terrorist as being one who engages in terrorism.
What else do we know about terrorists?
1) Terrorists do not have any "official" government support.
2) Terrorists do not have uniforms, rank or serial number to identify themselves with.
3) Terrorists do not carry their weapons openly.
4) Terrorists use military weapons and tactics and engage in warfare.
5) The primary targets of terrorism are civilian, not military.
The things that we know about terrorists seem to look very much like pirates. The definition of pirate is "one who engages in piracy." Piracy is defined as "Robbery on the high seas or in the air". So pirates are taking property by coercion. Terrorists are coercing people for political objectives.
Speaking for myself, this is a very fine distinction.