A 60-year-old man with severe depressive illness takes noticeably long pauses before answering even simple questions, moves slowly and with little spontaneous gesture, and describes feeling as though he is "moving through treacle." What is this?

This is psychomotor retardation: a generalised slowing of physical movement, speech, and thinking, with the classic subjective sense of being physically weighed down, exactly the pattern the phenomenon's own recognition features describe, and a recognised marker of depressive illness severity. It is not apathy, which the phenomenon's own text distinguishes as primarily a reduction in interest or concern rather than a physical and cognitive slowing. It is not catatonic stupor, which involves specific postural and motor phenomena rather than this smooth, generalised slowing, though severe retardation can shade into stupor. And it is not akathisia, which is a restless, drug-induced inner urge to move, the opposite pattern to the slowing described here.