The tendency to automatically mimic and synchronize facial expressions, vocalizations, postures, and movements with that of another person and, consequently, to converge emotionally.
Hatfield, E., Cacioppo, J. & Rapson, R.L. (1992). Emotional contagion. In Clark, M.S. (Ed.), Review of Personality and Social Psychology: Vol. 14. Emotion and Social Behavior. Newbury Park, CA: Sage.