Obsessionist

Part of speech: noun

Definitions

1. A person who is consumed by an intense, persistent fixation on a particular idea, activity, or object
2. Someone whose mind is held captive by a single all-consuming preoccupation that dominates their thinking
3. One who dwells compulsively on a certain subject to an extreme and often unhealthy degree

Etymology: "Obsessionist" is simply "obsession" with the suffix "-ist" attached. "Obsession" came into English from Latin "obsessio", a siege or blockade, derived from "obsidere", to besiege, and it was long used for a state of being beset by evil spirits before it came to mean a fixed, consuming idea. The suffix "-ist" marks a person who follows or practises something, as in "artist" or "novelist", so an obsessionist is one who is preoccupied, or who promotes an obsession. The word is a transparent formation, and its meaning follows directly from its parts. The notable point is the shift that "obsession" itself underwent: a term for military encirclement and possession by a spirit became, over time, a term for a mind that cannot let go of something.