What is a Microaggression?

OK, this is way off topic, but I asked my colleagues for clarification and have gotten almost no response. I guess (a) they do not care about the issue (odd), or (b) they think that I am just trying to stir up controversy (not on this occasion), or (c) they are, for whatever reason, hesitant to speak up on the issue (Why would this be?). One thing I know about our commentators here at SO is that they are not at all hesitant to speak up on controversial issues, so I will raise the question here: What is a “microaggression?” In academe we have been hearing a good bit lately about so-called microaggressions and how to prevent them. However, I am must not sure what they are. My fear is that unless we define microaggressions clearly and have definite, objective criteria for identifying them, in suppressing such alleged aggressions, we might censor or stifle the rough-and-tumble of free and open discussion.
Universities must stand for freedom of speech if they stand for anything, but free speech means that some people’s feelings are going to be hurt. It is impossible to allow freedom of speech and not to allow considerable freedom to offend. For instance, atheists in some of my philosophy classes might say things that would be offensive to conservative religious people—and vice versa—but I do not see how this can always be avoided in frank and open discussion. On VERY rare occasions I have had people in class discussions use hateful epithets like “fag” or other terms of derision. In those cases, I don’t have to say anything but just open the floor to the other students who then let the offender hear about it in no uncertain terms. But what is a “microaggression” and what is just a blunt (and perhaps uninformed) opinion does not seem clear to me. Consider the following statements that might very well be made in a class discussion:
“Black lives matter? Sure, but what we should say is that ALL lives matter.”
“No, I do not want a biological male in the same locker room with my teenage daughter.”
“Religion is just the opiate of the people, as Marx said.”
“It violates freedom of religion to require a devout Christian to make a wedding cake for a same-sex couple.”
“What is wrong with the term ‘illegal immigrant’? They are immigrants and they are here illegally.”
“Sharia law is opposed to American principles.”
Every day I read the letters to the editor of the Houston Chronicle, and statements like this are made there all the time. Sure, they are possibly uninformed or insensitive, but should we treat them as aggressions if they are voiced in class? Should we discourage such statements, or just allow other students to respond to them?
I am really genuinely curious and concerned. I want all of my students to feel welcome in my classes, but I also want them to feel the freedom to express their convictions frankly in open discussion. Any light readers can shed would be appreciated.