Origin: Latin suffix -sion
Obsession has 3 different meanings across 1 category:
an irrational motive for performing trivial or repetitive actions, even against your will
"her compulsion to wash her hands repeatedly"
A compulsive or irrational preoccupation.
"His obsession with checking the locks prevented him from ever falling asleep at night."
In plain English: An obsession is when you think about something so much that it takes over your mind and stops you from doing other things.
"His constant checking of his phone is becoming quite an obsession."
Usage: Use this term to describe an intense, often unhealthy fixation rather than a simple strong interest. It implies a loss of control that distinguishes the word from milder synonyms like "passion" or "preference."
The word obsession comes from the Latin obsessio, meaning "a besieging." It traveled into English through French, carrying with it the idea of being mentally trapped or surrounded like a city under siege.