Kinesics is the term used by sociologists, psychologists and anthropologists to refer to the interpretation of body language; facial expressions, posture, gestures, movements, etc. that humans use as a form of non-verbal communication.
This term was originated in 1952 by anthropologist Ray Birdwhistell during his study of filmed interactions between people that was intended to analyze different levels of communications. Birdwhistell estimated that "no more than 30-35 percent of human communication is carried by words", with the rest or communication being non-verbal.
By the time we become adults we are accustomed to identifying major body language cues that indicate discomfort, irritation, anger, flirtation, happiness, etc. in other people, but with study there are a great many other more subtle indications that can be identified.