"The common scholarly definition has two main criteria. The first says that the warring groups must be from the same country and fighting for control of the political center, control over a separatist state or to force a major change in policy. The second says that at least 1,000 people must have been killed, with at least 100 from each side."

Google {"civil war" +definition}, and you will find nearly all definitions with any specificity pretty much match this quote (from NY Times).