From the very helpful NYT article, listing various political terms, both left and right:
Alt-Right, Alt-Left, Antifa: A Glossary of Extremist Language
Alt-Left
Researchers who study extremist groups in the United States say there is no such thing as the “alt-left.” Mark Pitcavage, an analyst at the Anti-Defamation League, said the word had been made up to create a false equivalence between the far right and “anything vaguely left-seeming that they didn’t like.”
Some centrist liberals have taken to using this term.
“It did not arise organically, and it refers to no actual group or movement or network,” Mr. Pitcavage said in an email. “It’s just a made-up epithet, similar to certain people calling any news they don’t like ‘fake news.’”
On Tuesday, Mr. Trump said the “alt-left” was partly to blame for the Charlottesville violence, during which a counterprotester, Heather D. Heyer, was killed.
Another key thing about this bogus term: No one on the left identifies as “alt-left.” But people on the right, like those who marched with Tiki torches in C’ville, proudly identify as “alt-right.” They’ve chosen that term as their own, and have websites and media like Breitbart supporting it. The left doesn’t have an equivalent. And, as many others have mentioned, setting it up as an equivalent is deeply offensive for a host of reasons. As if people engaged in a battle against racism, xenophobia, misogyny and other forms of bigotry are equivalent to those who espouse them. As if people who push for Medicare for all, free college tuition, a living wage and are against neoliberal economics are somehow evil people, no different from Nazis and other fascist groups, etc. etc.
It’s one thing to hear this from right wingers. No surprise to see such ignorance and lack of historical knowledge coming from them, with their idiocy about “cultural Marxism” and so on. But it’s more than a shame to hear it coming from Democrats and those left of center, who should know better. They should know the history of the left and realize that those of us to the left of liberal have been around just as long as they have. They should remember when the terms left and right were first used, politically, back in 1789 in France, leftists existed then, too, and have a tradition of more than two centuries of passionate activism behind us. Politics has never been just about centrist Democrats and Republicans, and there is a ton of real estate to the left of the American Democratic Party. We’re not “alt.” We’re the actual left.
In my view, most of this seems to be a carry over from the highly unfortunate Sanders/Clinton food fights, and we should have moved beyond that absurd battle long ago. It destroyed this website and others for a time, and if this bashing of “the left” continues, I don’t see it ever coming back to pre-2016 energy levels again. If people really want the Democratic Party to win elections again, they won’t use terminology invented by the right used to divide and conquer it, and they’ll embrace those to their left, instead of endlessly blaming them for the results of this past election.
This article from Jacobin is a good read on the topic in question:
Burying the Lie of the “Alt-Left” — by Branko Marcetic. It's time to stop pretending that the same people fighting white supremacists are somehow exactly like them.