I feel like it's a very ill-defined term in the imperial core, but also there seems to be no coherent agreement on the left. Many thinkers have different definitions that often overlap.
Lenin called fascism "capitalism in decay"
Fanon called it "colonialism at home"
Umberto Eco offers his own incoherent mess of a definition
Roger Griffin defines it as a "palingenetic ultranationalism" that imagine a mythical "rebirth" of some previous glory (Rome, the volk, MAGA), and in doing so seek the "dominance of the insiders of the ultra-nation over those outside of it."
Parenti states that fascism "offers a beguiling mix of revolutionary-sounding mass appeals and reactionary class politics", adding that if fascism means anything "it means all-out government support for business and severe repression of anti-business, pro- labour forces."
Andreas Malm adapts Griffin's definition in White Skin, Black Fuel to a "palindefenIve, palingenetic ultranationalism", etc, adding that in addition to the sense of rebirth to some mythical glory time, there is also a mythical defense of the ultra-nation from those who are defined as foreign, be they Muslims, central American refugees, judeo-bolsheviks, etc.
I find the most functionally useful definition of fascism is Parenti's: the violent oppression of the left to maintain the dominance of the ownership class. However I feel like it lacks the element of violent chauvinism against arbitrarily defined others in society. That is to say I suppose I also lack a coherent definition.
What say you comrades?
To me fascism represents a mixture of the Parenti and Malm/Griffin definitions, at least for the historical examples we have: all have as a common trait the ultranationalism, the defense of the ultra-nation against the "other", and of course, pernicious, violent, ceaseless, unrepentant anticommunism.
This is not to say that these traits cannot be more fluid. America, as the forefront of the new fascist movement, now stands at an interesting crossroads between essentially two or three different flavors of fascism: either it embraces the GOP-style ultranationalism (to include its theocratic and MAGA/QAnon variants), or it embraces an inclusive, LGBTQ+, black-and-brown-positive, we're-all-equal-Americans type of fascism that more closely resembles that seen in the Veerhoven Starship Troopers film that the modern liberal consensus is pushing as the sole alternative to the GOP. Think of the MORE BLACK TRANS DRONE PILOTS and the "I'm just a smol bean CIA agent with general anxiety disorder UwU" tendency pushed forth and you might get a taste of what is to come. Both have different manifestations of how they express their violence, but they have one thing, without exception, in common: violent, pervasive anticommunism.