Having finished my internship at NATO and having just attended a conference on the (dis)integration of Europe, I have been thinking about (ir)regular fighters. Recent and not-so-recent events in Ukraine and other regions of South Eastern Europe demonstrate that the term 'insurgent' must always be defined in relation to a concrete situation of established authority, the history of that authority, and the degree of legitimacy that authority caries. As much as the term 'terrorist' relates to methods, and 'freedom fighter' relates to goals, 'insurgent' relates primarily to a positional (i.e. asymmetric) relationship vis-a-vis an opponent, i.e. an authority in the from of a government, an empire, etc.
Defining 'insurgent' only becomes problematic (in terms of political neutrality) when it is applied to a concrete set of actors. To avoid bias, one would have to determine empirically the identity of the legitimate authority. In some situations, such identifications will seem facile and remain uncontested, but it others such identifications will be one of the reasons for conflict and thus one of the issues to be resolved politically (or militarily).
Without expanding on what methods could be employed to determine 'authority' from 'insurgent', it will suffice to say here that insurgent is just as polemical a term as terrorist, if not more. If we look to Ukraine, all sorts of terms have been used in public discourse (NAZI, little green men, etc.). The term the most contested, however, is insurgent and the like (e.g. bandit, rebel). In a situation of contested legitimacy, where all actors are trying to win popular support, the term 'insurgent' becomes subjective and of little analytic use.
I continue to believe that one of the fundamental ways to improve the social sciences today is to improve the concepts used. One area where this is especially needed is in the study of political violence.