Microexpression
I was just reading an old Maxim, August 2000 if you were curious, which aside from predicting an attack by Osama bin Laden talks about this curious term in an article about spotting a liar.
I've read a book or two on readying body language and I really like this term. I always found that I had a knack for seeing behind what people do and say even before I read literature about it and I really like this term. It refers to the split-second change in facial features that shows the true thought pattern of a person. I think this is the kind of thing that I see subconsciously and it'll nag at me for awhile until I can figure out exactly what's going on.
Here's the formal expression given by good ol' wikipedia:
A microexpression is a brief, involuntary facial expression shown on the face of humans when one is trying to conceal an emotion. They consist of and completely resemble the six universal emotions: disgust, anger, fear, sadness, happiness, and surprise .