Here are a few conceptual attitudes that might be taken. First is an instance of ostensive definition. Pointing to the political and social experiences of Germany and Italy between 1930 and 1945, the theorist might say: "This is what I mean by fascism. These social formations, and any other historical examples that resemble them in important ways, are what I mean by 'fascism'." On this stance, nothing we discover about the cases becomes part of the definition of fascism. But these discoveries may become part of a social theory of violent social movements and political formations, and they will contribute to a causal understanding of these cases.
A second possibility is what we might call an operationalizing strategy, restricting ourselves to a thin and preliminary definition of the phenomenon of interest. The theorist looks at the case or cases that are the primary examples. He/she notes a few prominent characteristics -- use of violence against political enemies, an ideology based on resentment, and a vitriolic nationalism aimed against domestic minorities, perhaps, and might then say: "Operationally, I will classify societies [social movements, states, ideologies] as fascist based on these three criteria." And when it turns out that there are a very wide range of otherwise different examples that fall under these criteria, the theorist says: "I don't assert that all fascist societies, states, ideologies, or movements are fundamentally similar. If they share these 3 characteristics, they are fascist in my analysis."
A third approach might be an ordinary language approach: What are the connotations and presuppositions that "we" ordinarily have in mind when we use the word "fascist" in application to political behavior and structure? What do we mean by the language of "fascism"? A variant: how has the concept been used historically by earlier writers?
A fourth approach begins with a somewhat more reflective approach. The theorist notices that there were a number of rather similar but independent movements in the 1930s in Europe and Asia. He/she puts it forward that "Something similar was going on here." These parallel cases are instances of something -- call it fascism. My task is to ferret out what the real features of fascism are, as partially illustrated by these cases." This approach is essentialist. It assumes there is a hidden social reality that is pure fascism; that these cases imperfectly express that reality; and that the task of definition is to identify those underlying essential features. "This is what fascism really is; and once we've spelled out this theory of essential fascism, we will also understand the cases and their differences better."
A fifth possible perspective: These examples in Europe in the 1930s have many suggestive similarities. Take it as given that there is no "essence of fascism". But surely there were some similar forces, events, structures, and processes that combined conjuncturally to bring about the intertwining similarities witnessed in Germany, Romania, Italy, Spain, and (with different outcomes) Britain and France. The theorist expresses herself this way: "I will formulate an articulated representation -- model -- of my best thinking about the causal and social features that seem most important in these processes. That model is my "definition" of fascism. It is an "ideal type." But it doesn't pretend to capture the underlying essence. It instead serves as a guide for empirical and historical research. It is a substantive set of hypotheses about how these complex examples worked. It is intended to guide careful historical comparisons and, eventually, corrections and revisions of my current thinking about how fascisms worked."
A sixth perspective might downplay the importance of framing a specific concept of fascism altogether. This is the "no definition" approach. The historian-sociologist might say: "Violent, anti-democratic, and surprising things happened in Europe between the wars. I want to put together some best-thinking from the social sciences to identify and understand the processes and structures that were underway in those years that combined to create these violent and anti-democratic political developments. Call it the period of fascisms if you like; my goal is to understand lower-level political and social processes and structures that created this conjuncture." These processes may have to do with resource mobilization and social movements; theories of class politics; theories of reactions to crisis; theories of communication; theories of political opportunism; theories of economic structure, trade, colonial policies, etc. And comparison across the positive and negative cases will help to refine our understanding of how those processes worked and what their limits and conditions were.
Several of these approaches are essentialist: they presuppose that fascism is a discrete phenomenon that can be specified in a carefully drawn set of necessary and sufficient conditions. Several other approaches are nominalistic, in that they do not presuppose that the term really refers to a coherent underlying social reality. Our understanding of reality is limited to concrete ideas about processes, structure, and forms of agency that we can study and analyze; the "big" concepts only pull together related sets of those processes and structures into loose configurations. And one, the ordinary language strategy, is purely semantic. It simply tries to explicate the concept of fascism as it is used by competent users of the term. What do they mean to convey?
Several things seem clear to me. First, we can't capture a complex social reality like fascism or democracy through a definition. The definition serves only to focus our attention on a particular range of social phenomena. But to actually know anything about those phenomena we have to investigate them historically and empirically. Second, historical concepts like fascism do not single out parts of the social world in the way that natural kind terms single out discrete parts of the natural world. Fascism is not a natural kind that is fundamentally the same through its many instances. Third, theoretically and historically developed models of fascism are genuinely useful. Call these detailed historical constructs; or perhaps, call them ideal types. But these constructs are empirically based: they are fallible; they reflect the researcher's hunches or stereotypes about how this sort of stuff works; and we must recognize from the start that we will encounter instances that don't fit the theoretical construct. This kind of historically detailed and articulated construct of fascism is useful precisely because it leads us to examine non-standard cases carefully. The non-standard case can point up exactly the ways in which the construct is a heuristic organizing device, rather than a way of organizing every thing we know about fascism.
So here is the introductory paragraph I'd like to see in a comparative historical study of European fascisms:
The inter-war period saw a number of social movements, conflicts, and power regimes that emphasized nationalism, violence, and interpersonal resentment. Some of these countries went on to form authoritarian states; others did not. My goal in this study is to search out the causes, conditions, and structures that appear to have played an important role in the rise of these movements in some countries; the factors that helped these movements to seize power in several countries; and the factors that prevented the seizure of power in yet other countries. There is variation across all the cases, in ways that may be more or less important. The societies where these movements seized power are often characterized as "fascist", but not much turns on the name. My purpose here is simply to identify the large currents that seem to have been influential in this turbulent time. In this way I agree with McAdam, Tarrow, and Tilly in their studies of contentious politics: it is not the high-level concepts like war and revolution that shed light, but rather the specific meso-level causal and ideological processes where we can really learn something important about how societies come to embody and change with these kinds of politics.