So-called antifa protesters and counterprotesters have drawn considerable attention of late, after the deadly violence this month surrounding a white nationalist rally in Charlottesville, Virginia.
"Antifa" is short for anti-fascist, or someone opposed to fascism, which the Oxford English Dictionary defines as "an authoritarian and nationalistic right-wing system of government and social organization."
Many people who demonstrate against President Trump consider him a fascist, although others do not, such Gianna Riotta, an Italian journalist who argued her case in a recent Atlantic essay.
Though some have referred to antifa as a group, it's more akin to a cause or a phenomenon, according to protesters and organizers who spoke with ABC News.
Many of the protesters who arrived in Charlottesville on Aug. 12 to oppose the white nationalist rally hailed from a swath of left-leaning groups. Some of those groups are viewed as anti-fascist and opposed to white nationalism but do not consider themselves antifa.
Here's what you should know about the term:
How did 'antifa' originate?
Historians point to Germany as the source of the term. The socialist publication Jacobin wrote about the origins of "antifa" in May, suggesting that men and women from Germany's socialist labor movement who were part of an illegal resistance to the Nazis helped popularize the word in the aftermath of World War II. These resistance movements became formal political organizations with names like Antifaschistische Ausschüsse, Antifaschistische Kommittees and Antifaschistische Aktion, according to Jacobin.
They were called "antifa" for short, a name that came to mean opposition to fascism from a leftist perspective.
The term has gained notoriety this year partly because of responses to the Trump presidency and vocal opposition to his policies.
"Right now, we're in a very dangerous place," Lacy MacAuley, a self-described antifa activist, told ABC News' "20/20" this month. "We're in a very troubling place."
Does the perceived link between antifa and violence match the reality?
This is tricky: Some antifa activists ascribe to violence as a solution to combating the perceived threat of fascism, while others do not.
It's difficult to read right-leaning publications or social media accounts today without coming across the word "antifa" in association with acts of violence, such as using pepper spray against white nationalists and destroying property.
These have typically been so-called black bloc protesters, or people who don black masks and clothing at rallies and support violence to combat what they perceive to be the threat of fascism.
Black bloc protesters are frequently associated with anarchism, as opposed to socialism or communism.
The Nation, a progressive weekly magazine, wrote in a January 2017 story about the protests that surrounded Trump's inauguration, "The black bloc is not a group but an anarchist tactic — marching as a confrontational united force, uniformed in black and anonymized for security. Once deployed, the tactic has an alchemic quality, turning into a temporary object — the black bloc."
The distinction between black bloc and someone who simply opposes fascism at a protest is sometimes lost in media coverage, said protesters and organizers.
Jabari Brisport, who is running as a Green Party candidate to represent New York City's 35th Council District, attended the counterprotest in Charlottesville with a Black Lives Matter group. Fox News referred to Brisport as an "antifa protester," but he told ABC News that, while it's true that he opposes fascism, he was primarily there to support BLM.
"I arrived with a contingent from Black Lives Matter to represent ourselves and tell the world that our lives matter," he told ABC News.
"Antifa" has been given a "militant" connotation, he said, adding that such a description fits only a minority of the protesters who were there.
Maria Svart, the national director of Democratic Socialists of America, which says it has seen a large uptick in membership since Trump's inauguration, told ABC News that members of her group were in Charlottesville explicitly to protest against fascism and white supremacy and that they ascribe to a policy of nonviolence.
"We're 100 percent committed to a nonviolent political revolution," read a statement the organization sent to ABC News.
Who identifies with antifa?
The answer is unclear: Many protesters might consider themselves to be against fascism but identify as members of various political groups or other organizations, such as Black Lives Matter, according to people who attended the counterprotests in Charlottesville.
One community organizer, who joined the Charlottesville counterprotests and asked not to be named, told ABC News that the counterprotesters included members of Black Lives Matter, the Democratic Socialists of America, the Industrial Workers of the World, the anti-racism group Showing Up for Racial Justice, the International Socialist Organization, legal aid groups, smaller left-wing groups and the clergy.
Unaffiliated anarchists and black bloc anarchists made up only a small fraction of the counterprotesters, the organizer said, but the majority of counterprotesters would consider themselves anti-fascist, even if not specifically aligned with antifa.