(CNN) President Donald Trump equated white supremacist marchers in Charlottesville to counterprotesters by calling them "alt-left," prompting an immediate backlash and questions about the term itself.
"What about the alt-left that came charging at, as you say, the alt-right, do they have any semblance of guilt ... What about the fact they came charging with clubs in hands, swinging clubs? Do they have any problem? I think they do," Trump said at a news conference Tuesday referring to a rally in Charlottesville that turned violent over the weekend.
Many people have never heard of the "alt-left," but "alt-right" is a term that's found its way into popular culture and the vernacular of politicians and journalists.
Richard Spencer, president of the National Policy Institute and the editor of Radix Journal, is credited with coining the term "alt-right."
"I don't use the term white nationalist to describe myself," he said. "I like the term alt-right. It has an openness to it. And immediately understandable. We're coming from a new perspective."
But Oren Segal, director of the Anti-Defamation League's Center on Extremism, said this is simply a rebranding -- "a new name for this old hatred."
It's a "made-up term" used by people on the right to "suggest there is a similar movement on the left," Segal said.
But there's no equivalent with the anti-Semitic and bigoted groups that call themselves "alt-right", he said.
George Hawley, an assistant professor of political science at the University of Alabama, said the "alt-left" term has been most aggressively pushed by Fox News Channel's Sean Hannity, but it's not a label anyone or group has adopted for themselves.
"There is no such movement as the alt-left. Obviously, there are left-wing extremists but there is no congruence between the far-left and the alt-right."
Antifa, which is short for anti-fascist, is a controversial opposition group formed by a broad group of people whose political beliefs lean toward the left but do not conform with the Democratic Party platform. Members are known for causing damage to property during protests and some employ radical or militant tactics to get their message across.
"There is violence on the left. The anti-fascists engage with those they oppose through physical confrontation. And that is a problem. That is an extremists tactic. There is also bigotry on the left," Segal said.
One reason could be that left groups are not trying to repackage themselves to go mainstream as the far right has, he said.
"Do not get me wrong; there are extremists on the left, including very problematic ones. But the "alt left" is not a thing. It's just an insult," wrote Mark Pitcavage, a senior research fellow at the ADL center, on Twitter.
"Alt-right" has become intertwined with the term white nationalism, which originated as a euphemism for white supremacy, the belief that white people are superior to all other races and should therefore dominate society, Segal said.
People who hold these beliefs sometimes go by other names, including alt-right, Identarians and race realists.
Other white supremacist groups include the Ku Klux Klan and neo-Nazis. But most white supremacists aren't affiliated with an organized group, Segal said. Some also take measures to distance themselves from known hate groups, like the KKK.
Some Klansmen still favor white robes and hoods. But groups like the alt-right, overwhelmingly made up of Millennial men, prefer khakis and collared shirts. They're active on social media and employ irony and humor in their messaging, Segal said.
But the goal is the same: a white ethno-state where each race lives in a separate nation.
White supremacists and their ilk see diversity as a threat, Segal said. A popular white supremacist slogan is, "Diversity is a code word for white genocide."
A year ago, Republican presidential candidate Trump pushed back at Hillary Clinton's characterization of him as a supporter of the "alt-right."
"There's no 'alt-right' or 'alt-left.' All I'm embracing is common sense," Trump said at the time. "We're bringing love."
On Tuesday night, members of Congress and celebrities took to social media to criticize how Trump compared counterprotesters and neo-Nazis in Charlottesville.
"No, not the same. One side is racist, bigoted, Nazi. The other opposes racism and bigotry. Morally different universes," Mitt Romney, the 2012 Republican nominee and former Massachusetts governor wrote on Twitter.
Massachusetts Democratic Sen. Elizabeth Warren called Trump's comments sick.
"The President of the United States just defended neo-Nazis and blamed those who condemn their racism and hate. This is sick," Warren wrote.
Bernie Sanders tweeted, "The violence in Charlottesville was not caused by the "alt-left," (whatever that may be). It was caused by Neo-Nazis and white supremacists."
Star Trek actor and activist George Takei condemned Trump's use of the term "alt-left."
"Trump's trying to invent an "alt-left" as scary as Nazis. But if standing up to white supremacy makes someone alt-left, then count me in," Takei wrote.