Wiktionary
n. A fleeting, involuntary facial expression that may reveal private thoughts or emotions.
Wikipedia
A microexpression is a brief, involuntary facial expression shown on the face of humans according to emotions experienced. They usually occur in high-stakes situations, where people have something to lose or gain. Microexpressions occur when a person is consciously trying to conceal all signs of how they are feeling, or when a person does not consciously know how they are feeling. Unlike regular facial expressions, it is difficult/impossible to hide microexpression reactions. Microexpressions cannot be controlled as they happen in a fraction of a second, but it's possible to capture someone's expressions with a high speed camera and replay them at much slower speeds. Microexpressions express the seven universal emotions: disgust, anger, fear, sadness, happiness, contempt, and surprise. Nevertheless, in the 1990s, Paul Ekman expanded his list of emotions, including a range of positive and negative emotions not all of which are encoded in facial muscles. These emotions are amusement, contempt, embarrassment, anxiety, guilt, pride, relief, contentment, pleasure, and shame. They are very brief in duration, lasting only 1/25 to 1/15 of a second.
Usage examples of "microexpression".
Most people would have missed it, but his Secret Service training to detect what scientists referred to as microexpressions, the subtle and almost imperceptible facial cues that subjects unknowingly give off when they are not telling the truth, made it clear to Harvath that the man was lying.