Who has defined fascism as such? How do the practices of Stalinism root in Italian authoritarianism?

Defining fascism as any form of authoritarianism broadens the term so much as to render it useless.

It’s useful to be able to talk about the ways in which the ideologies which governed Franco’s Spain and Mussolini’s Italy are more similar to each other than say, something like the DPRK under Juche. If we want to refer to something as authoritarian, we already have the word authoritarian.