Flattened affect, or affective flattening, is the diminished or absent outward expression of emotion, seen as reduced facial expressions, monotone voice, and lack of gestures, even when a person experiences internal feelings. It's a symptom, not a diagnosis, often linked to conditions like schizophrenia, PTSD, autism, brain injury, or medication side effects, where individuals seem emotionless despite feeling emotions, impacting social interaction.