What is “Antifa”?

In June 2020, United States President Donald Trump posted a Twitter update stating his wish to designate Antifa as a terrorist organization; this happened at the height of the global protests against police brutality and racial injustice, and it also took place in the backdrop of the coronavirus pandemic. For many Americans, this was the first time they heard Antifa mentioned in a political context, and Trump's tweet suggested an established structure complete with leadership and active chapters.