Your example is interesting, because 'terrorism' is a rather ill-defined term - i.e. some people refer as 'terrorists' to what other people refer to as 'freedom fighters'. When is the former making a false claim?
A quote like "They call him freedom fighter, I call him terrorist" is clearly an opinion and yes, it makes a difference whether you use the word in the sense of a legal definition or an abstract concept or a hyperbolic metaphor. The level rod is dignity.
Terrorism is well defined, by Alex P. Schmid in 1988:
Schmid and Jongman (1988): "Terrorism is an anxiety-inspiring method of repeated violent action, employed by (semi-) clandestine individual, group or state actors, for idiosyncratic, criminal or political reasons, whereby - in contrast to assassination - the direct targets of violence are not the main targets. The immediate human victims of violence are generally chosen randomly (targets of opportunity) or selectively (representative or symbolic targets) from a target population, and serve as message generators. Threat- and violence-based communication processes between terrorist (organization), (imperilled) victims, and main targets are used to manipulate the main target (audience(s)), turning it into a target of terror, a target of demands, or a target of attention, depending on whether intimidation, coercion, or propaganda is primarily sought".
As well as by "The Revised Academic Consensus Definition of Terrorism (2011)" by Alex Schmid available at [1]
The problems derive from definition being complex. Examples: laymen don't have the analytical power, powers that be have a conflict of interest as you describe. However that is certainly not the same as terrorism being an ill-defined term.
The revised definition is used world wide by terrorism experts such as Prof. Dr. Beatrice de Graaf and Prof. Dr. Edwin Bakker.
If this subjects is of interest to you I recommend the Coursera course "Terrorism and Counterterrorism: Comparing Theory and Practice" [2] by Prof. Dr. Edwin Bakker.
[1] http://www.terrorismanalysts.com/pt/index.php/pot/article/vi...
In the American Legal system at least, both domestic and international terrorism are very well defined terms: