Rant

This opinion is a rant—a passionate, typically angry, speech or piece of writing about one particular topic.

This rant is about American anti-maskers—people who are opposed to wearing a face mask during the coronavirus outbreak. 

A "rant" is passionate, typically angry, speech or writing about a particular topic.

The topic of this rant from 2013 is the failure of marketing schools to prepare students for the new internet-based marketing industry.