There is no "alt-left". It's a term made up by the right to far-right in an attempt to create a false equivalence between white supremacists and those opposing white supremacism:

> **It's a "made-up term" used by people on the right to "suggest there is a similar movement on the left**," [Oren Segal, director of the Anti-Defamation League's Center on Extremism,] said.

> But there's no equivalent with the anti-Semitic and bigoted groups that call themselves "alt-right", he said.

> [George Hawley: ] "**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."<sub>[CNN: What's the 'alt-left'? Experts say it's a 'made-up term'](http://edition.cnn.com/2017/08/16/politics/what-is-alt-left/index.html)</sub>

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> 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.”**<sub>[New York Times: Alt-Right, Alt-Left, Antifa: A Glossary of Extremist Language](https://www.nytimes.com/2017/08/15/us/politics/alt-left-alt-right-glossary.html?mcubz=1)</sub>

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> Ultimately, **the intent seems to be to frame alt-left as the opposite of alt-right and create a false equivalence between groups on the far ends of the right and left**. But here's the thing: No left-wing group has ever called itself the alt-left. And the groups smeared by the alt-left label don't include anything like the heinousness of overt white supremacism that has increasingly defined the alt-right.

> It's a blanket term some right-wing media commentators and white nationalists have taken to throwing over groups they disagree with <sub>[Wired: There Is No 'Alt-Left,' No Matter What Trump Says](https://www.wired.com/story/what-is-alt-left/)</sub>

"Alt-right" on the other hand is a self-applied term that was made up by white supremacists themselves for propaganda purposes. 

There are of course far-left ideologies such as various sorts of communism, anarchism, etc. They are not comparable to the so called "alt-right" though; while some may contain racism or antisemitism, it is not central to any left-wing ideology, while they are central elements of the so called "alt-right". There are of course a lot of other differences, but that seems to be outside of the scope of this question.