Facial Action Coding Scheme (FACS)

A coding system that distinguishes all possible visually distinguishable facial movements based on a comprehensive analysis of the anatomical basis of facial movement.

Reference:
Ekman, P., & Friesen, W. V. (1976). Measuring facial movement. Environmental Psychology and Nonverbal Behavior, 1(1), 56-75. doi: 10.1007/BF0111546.