apathy; indifference; listlessness
General lack of emotion or interest; can describe a person's state or an atmosphere.
彼は無感情な顔でその知らせを聞いた。
He listened to the news with an apathetic expression.
無感情な態度が周囲を不安にさせた。
His indifferent attitude made those around him uneasy.
athymia
Medical term for a loss or absence of emotion; used in psychiatric contexts.
Athymia is known as one of the negative symptoms of schizophrenia.
無感情 means lacking emotion or feeling, while 無反応 means lacking any observable reaction, which can happen even when emotion is present.
Compound of 無 (mu, 'without') and 感情 (kanjō, 'emotion').