The works of Georgi Dimitrov are the go-to for defining fascism hands down.

"Fascism is not a form of state power "standing above both classes – the proletariat and the bourgeoisie," as Otto Bauer, for instance, has asserted.

It is not "the revolt of the petty bourgeoisie which has captured the machinery of the state," as the British Socialist Brailsford declares.

No, fascism is not a power standing above class, nor government of the petty bourgeoisie or the lumpen-proletariat over finance capital. Fascism is the power of finance capital itself.

It is the organization of terrorist vengeance against the working class and the revolutionary section of the peasantry and intelligentsia. In foreign policy, fascism is jingoism in its most brutal form, fomenting bestial hatred of other nations.... >

The development of fascism, and the fascist dictatorship itself, assume different forms in different countries, according to historical, social and economic conditions and to the national peculiarities, and the international position of the given country."

Give The Fascist Offensive and the Tasks of the Communist International, a read

Eco's analysis lacks fascism's focus on anticommunism, which is the whole reason capital first tolerates and later embraces fascism. This is the defining characteristic, especially if you try to parse out what separates fascism from a mere dictatorship or an absolutist monarchy.

There are also some points he makes that are things everyone does to varying degrees. You can drop the "the enemy is both too strong and too weak" on anyone who talks about the strengths and weaknesses of an opponent, for example.

Blackshirts and Reds by Michael Parenti is fantastic if you haven't read it