Essential: The items may share a common defining characteristic (e.g. "liquid", "metal")
-- Etiological: The items may share a common cause (e.g. "viral illness")
-- Structural: The items may share a common underlying structure (e.g. "protein", "elm")
-- Functional: The items may share a common function (e.g. "weapon", "school")
Non-essential: The items may lack necessary and sufficient conditions but share some overlapping characteristics.
-- Symptomatic: The items may share a set of observable characteristics or symtoms (e.g. "pneumonia", "schizophrenia")
-- Cluster: the items may share some among a list of characteristics (e.g. "game", "leader")
(It is a bit curious to observe that this amounts to a classification of systems of classification.)
Now let's consider some social science terms and see where they fall in this scheme: riot, civil war, bicameral legislature, ethnic group, democracy, charismatic leader, financial city, working class organization.
Several of these concepts are symtom or cluster concepts: riot, democracy. Several others are etiological or structural: civil war, bicameral legislature, financial city. One combines structural with functional criteria: bicameral legislature.
Now suppose we have identified a set of things as being "democracies". They share some among a set of features that are associated with democracy, and there is no set of features shared by all instances. What kinds of social science inquiry can we do? First, we can do comparisons within the group of democracies we have identified; we can look for similarities and differences across the group. And second, we can ask whether membership in this group is associated with membership in some other group beyond what chance would predict. In other words, we can consider whether there are true statements like "all democracies are X" or "most democracies are Y."
It is not the case that "all pneumonias respond to penicillin"--for the reason that there are two causal and structural kinds of pneumonia, and only one of these involves organisms treatable with penicillin. The causal heterogeneity of this group means that strong generalizations are difficult or impossible here.
The concepts of riot, revolution, and democracy are similarly heterogeneous, both causally and structurally. So we should expect only weak generalizations across this group and other social charactistics. On the other hand, the tools of social comparison are most valuable here. We can discover through additional comparative work within the category, whether there are similar structural and causal processes at work among instances of this concept.