A microaggression is a very small aggression, usually racial or sexuality-based, or even sexually based. For example, it's when you're talking to people from a different background and you mention that they speak good English or that they're accent isn't that strong. Any way to “other” somebody is a microaggression. Asking black people if you can touch their hair and things like that are microaggressions. There are a lot of examples.
On November 21st, 2016. See this statement in context.