I wrote: "it's not just an idea."

Antifa is a political movement in Germany composed of multiple far-left, autonomous, militant groups and individuals who describe themselves as anti-fascist. According to the German Federal Office for the Protection of the Constitution and the Federal Agency for Civic Education, the use of the epithet fascist against opponents and the view of capitalism as a form of fascism are central to the movement. The antifa movement has existed in different eras and incarnations, dating back to Antifaschistische Aktion, from which the moniker antifa came. It was set up by the then-Stalinist Communist Party of Germany (KPD) during the late history of the Weimar Republic. After the forced dissolution in the wake of Machtergreifung in 1933, the movement went underground. In the postwar era, Antifaschistische Aktion inspired a variety of different movements, groups and individuals in Germany as well as other countries which widely adopted variants of its aesthetics and some of its tactics. Known as the wider antifa movement, the contemporary antifa groups have no direct organisational connection to Antifaschistische Aktion.

Antifa (/ænˈtiːfə, ˈænti(ˌ)fə/) is a left-wing anti-fascist and anti-racist political movement in the United States. It consists of a highly decentralized array of autonomous groups that use nonviolent direct action, incivility, and violence to achieve their aims. Most antifa political activism is nonviolent, involving poster and flyer campaigns, mutual aid, speeches, protest marches, and community organizing. However, some who identify as antifa also use tactics involving digital activism, doxing, harassment, physical violence, and property damage. Members of antifa aim to combat far-right extremists, including neo-Nazis and white supremacists.

Antifa can be understood as a shortened form of the word anti-fascist, and like that word, it's both an adjective and a noun. The adjectival use is straightforward ("antifa protestors/groups" are protestors/groups opposed to fascism), but the noun use is more complex. Antifa the noun can refer to:

an anti-fascist movement: Antifa is the backlash to the backlash, a defensive response to the growing presence of right-wing extremism. — Todd Gitlin, The New York Times, 28 Aug. 2017

an anti-fascist person: The groups … prominently featured about 100 Christian ministers in clerical garb, angry Charlottesville residents, peace advocates, Black Lives Matter activists, and self-styled anti-fascists who call themselves "antifas" …. — Peter Weber, The Week, 15 Aug. 2017

a particular group of anti-fascists: After [World War II], Antifas varied in size and composition across the former Reich, now divided into four zones of occupation, and developed in interaction with the local occupying power. — Loren Balhorn, Jacobin, 8 May 2017

or to anti-fascists generally: Berkeley—a hotbed of activism of on both sides of the political divide since the 1960s—has been the scene of much of Antifa’s activities over the last year. — Andrew O'Reilly, Fox News (foxnews.com), 30 Aug. 2017

The term isn't new. In fact, it was first proposed for entry by one of our lexicographers in 1955, but it wasn't common enough at the time to justify its inclusion. Times, of course, have changed, and the word is now a very good candidate for entry, as the examples above demonstrate.

Our English word antifa is a German borrowing, but the word's story starts in Italy. The fa of antifa has its roots in the Italian word fascio (plural fasci), a word that means literally "bundle," and figuratively "group"; as early as 1914, Italians were using a derivative of fascio, fascisti, to refer to members of a political fascio. The fascisti adopted as their emblem an insignia of official authority in ancient Rome with a linguistically very appropriate name: the bundle of rods with an ax head projecting up from it known as the fasces. By 1919, the word fascisti had as its referent the black-shirted members of Benito Mussolini's "combat groups" in particular.

(Sources - Wikipedia and https://www.merriam-webster.com/wordplay/words-were-watching-antifa)