A very short facial expression of an intense, concealed emotion. Also: microexpression, micro expression.
Ekman's main scientific contribution has been to show how the face is the mind's involuntary messenger. Even when suppressed or subconscious, emotions make fleeting appearances. These micro-expressions are as brief as one-one-hundred-twenty-fifth of a second. They are too fast for most people to recognize, but they lay bare our true feelings.
To the untrained eye, such micro-expressions may only be visible if slowed down on film. But after an hour of training, Ekman says, even a novice can begin to detect them in real time.
Laura Kurtzman, "How faces mirror emotion," San Jose Mercury News, April 29, 2003
What's the secret? In a word, microexpressions. If you can learn to catch these superfast facial expressions they come and go in less than 40 milliseconds then you have the clues you need. We are barely aware that we make microexpressions, and they are over so quickly that they are very difficult to fake. That makes them very good indicators of true feelings, but few people ever manage to detect them.
"The word liar," New Scientist, March 29, 2003
Psychologist Paul Ekman ran the film over and over until he found the clue. Mary, a housewife who had attempted suicide three times and had been confined to a mental institution, appeared chipper and confident onscreen as she asked her doctor for a weekend pass. Her interview, secretly shot for research purposes, was so convincing that Mary got the pass, but she subsequently admitted that she had been lying and had wanted to get away for another suicide try. By slowing down the film, Ekman found that Mary's face had sagged into despair, a telltale "microexpression" that lasted only one twenty-fourth of a second. Later he found other quick movements of deceit: part of a hand shrug, the brief lift of a shoulder.
John Leo, "The Fine Art of Catching Liars," Time Magazine, April 22, 1985
The "Ekman" mentioned in the first example citation is psychologist Paul Ekman, of the University of California, San Francisco, who invented the phrase micro-expression back in 1985. He was much in the news earlier this year because in April he published a book titled Emotions Revealed: Recognizing Faces and Feelings to Improve Communication and Emotional Life.