I can't tell you how much reading I have done on this subject, but I think the best definiton for insurgency is the one Bard O'Niell uses in Terrorism & Insurgency: Insided Modern Revolutionary Warfare.

Insurgency may be defined as a struggle between a nonruling group and the ruling authorities in which the nonruling group conciously uses political resources (e.g. organizational expertise, propaganda, and demonstrations) and violence to destroy, reformulate or sustain the basis of legitimacy of one or more aspects of politics