Fascism, in emic terms, is an idealist ideology that configures the state as a conjugation of the heterogeneous and supposes the negation of liberalism's self-conception. In etic terms, it is the political form that capitalism took during the crises of the interwar period in Europe.
Idealist → Fascism is idealistic, it positions itself within the idealism-materialism philosophical debate in the idealism side. Ideas are the primary form in which things exist, and they exert influence on the real/material world.
State → Instrument of class oppression, the arm with which one class keeps itself in power over another class.
Conjugation of the heterogeneous → Imagine the fasces, the symbol the Italian fascist party took for itself. It is a bundle of sticks which hold an axe. Each stick on its own is brittle, but in the fasces it is very hard to break. Fascism seeks to make the state into a fasces. It negates the equality of humans, fundamentally. Workers, aristocrats, capitalists, artists, peasants, artisans, so on are differenciated groups. Within the state, these different groups are united and become able to impose their common will. Fascism holds that these different groups must be subsumed to the state because it is only then that they're free. Alone, outside of the commonality, they're brittle sticks that break easily.
As a small note, some currents of fascism will substitute the state for race, or nationality, or any of these kinds of categories. The point is that they rally around one commonality under which a population of unequals is able to effect its will. This commonality is necessarily exclusive to the group(s) which is(are) perceived to threaten the freedom of the people within the common category. In practical terms, this is always achieved through the state
Fascism negates the equality of men but it affirms that this does not devolve into chaos, rather that it is possible to organize the internal strata of society into a coherent whole that overcomes the stasis of society.
Stasis → A greek term that means "civil war", it refers to the moment of internal struggle in a state that prevents its normal course. Through a marxist lens it is the high point of class war, the materialization of the accumulation of internal contradictions, when the two main classes in a society take up arms against eachother, and the state's eutaxy is broken. Here again we see the relation between fascism and capitalist crises.
Negation of liberalism's self-conception → Fascism is only necessary for the radical reconfiguration of the relations of production in capitalism (more on this below). Consequently, it presents itself as a break from the liberal status-quo. The original fascist movements presented themselves as revolutionary and borrowed much of the rethoric used by coetaneous communists. It addresses the common gripes that arise with the liberal administration of capitalism in times of crises. To justify such a radical change, it appears as a polar opposite to the principles of liberalism. The phrase "neither left nor right" and general third-positionism represents this. It refers to the spectrum of politics in a capitalist state, the totality of liberalism, the common principles of every liberal current.
The political form that capitalism took during the crises of the interwar period in Europe → It is commonly said that fascism is capitalism's last response to deep crises. I think that, at the current stage of research on the current state of capitalism, its trajectory, and the actual relation between it and current fascist movements, it is hard to make a comparison with this concrete time and place, its conditions and political landscape.
You can identify that capitalism worldwide is facing a crisis, the political and economic changes we see are the live adaptation of the infrastructure-supraestructure to this recessionary cycle However, I'd argue that there is an important piece missing from the comparison to the interwar, which is the resistance capitalism finds when it flows from reform to reform. Compared to the interwar in Europe, the working class now are barely organized Internationally, there is nothing even similar to the USSR. No matter how positively or negatively you view China and its economic attributes, it does not embody the solid wall against capitalism that the USSR did, they have a different foreign policy.
The quantitatively and qualitatively declined state of the organization of the working class today means that the reconfigurations capitalism is forced to take before the crisis can be done through the normal channels of the capitalist state without an opposition as massive or effective as existed in Europe in the interwar. As such, the direct and highly antagonizing control fascism takes over the state is not necessary, at least to the degree it was needed in the interwar. Ultimately, it lead to the genocidal invasion of the USSR by the alliance named after anti-communism, whereupon 5.1 million laid down their lives for capitalism, and 26.7 - 34 million laid down their lives for socialism. It is simply better for the health of the capitalist system to avoid such bloodshed if it is able to readjust without too much resistance.
Hence, I don't think it is yet possible to define fascism in a way that meaningfully groups the interwar period's movements and all the post-1945 "fascist" movements, such as the US-imposed dictatorships in Latin America and the myriad of minority, non-exalted "fascist" movements spread throughout the central countries of the imperialist pyramid. Perhaps it will be possible in the future, with the ease only hindsight can afford.