Amimia refers to a loss of mimesis, or of the ability to express emotion, whether through voice or facial expression or gestures. Patients with this symptom generally have masked facies (that is, a facial expression that is as fixed and unchanging as a mask) and speak in a monotone. Schizophrenics and Parkinson's patients suffer from amimia, though in the latter it's secondary to their akinesia--they may feel a full range of emotions that they just can't express.

This usage of the term might seem a bit inaccurate, given that mimesis generally refers to imitation. My suspicion (though I can't confirm it) is that the usage arose because neurologists sometimes ask patients to imitate gestures, facial expressions, vocal inflections, and so forth, and these patients were incapable of doing so.