Fascism was a political philosophy originating on Earth. It was the government style of the Nazi state in Germany in the 1930s, and of the Terran Empire in the Mirror universe. It was also imported to Ekos. (TOS: "The City on the Edge of Forever", "Patterns of Force"; DIS: "Despite Yourself").
A key pillar of fascism was the so-called "Leader Principle" which stated that a government should have a single leader as its head, with all others obeying this leader without question. In the 20th century fascist leaders were generally deemed evil and psychotic men and it was for this reason that fascism failed on Earth as a viable form of government. However, even in cases where a fascist leader governed for the good of the people, it was a fact that when any man held so much power then it was in turn irresistible that an urge to "play God" would corrupt the government. Such was the case on Ekos, where Federation historian John Gill founded a peaceful fascist government only to have his government turn evil just like its predecessors. (TOS: "Patterns of Force")
Quark called Odo a fascist when the two got into a fight after crashing on an uninhabited Class L planet in 2373. (DS9: "The Ascent")
Ron D. Moore also described Odo's philosophy as "latent fascism". (AOL chat, 1997)
In a series of visions taking place in 1950s Earth which Benjamin Sisko experienced in 2374, Herbert Rossoff accused Douglas Pabst of being a fascist. (DS9: "Far Beyond the Stars")
in 2381, Shaxs referred to the Cardassian occupation of Bajor as fascist. (LD: "wej Duj") Later that year, he referred to Deep Space 9 as a "tacky Cardassian fascist eyesore". (LD: "Hear All, Trust Nothing")
The structure of the Cardassian Union was very reminiscent of historical fascist empires, with its extreme devotion to the state and the Cardassian Articles of Jurisprudence, which assumed guilt prior to trials.
Cristóbal Rios referred to the Confederation of Earth from an alternate timeline as fascist. (PIC: "Assimilation")