A microexpression is a tiny facial expression that lasts less than a quarter of a second. They often occur involuntarily, and can reveal emotions people are trying to hide, or may not even be aware of themselves.

Most people do not seem to perceive microexpressions in themselves or others. However, in the Diogenes Project, Paul Ekman found that these tiny movements often can expose lying, and that a very, very small percentage of those he studied had a preternatural knack for detecting them. He now claims that anyone can be trained to see such microexpressions relatively easily.

Microexpressions are a kind of nonverbal communication.