Obsession
In short
An intense and persistent preoccupation with a specific person, idea, or activity that dominates a person's thoughts.
In plain words
It is when you cannot stop thinking about one thing. This feeling is so strong that it makes it hard to focus on anything else in your daily life.
A simple example
A person might spend every waking hour researching a single historical event. They might neglect their sleep, work, and friends just to find more information.
Why it matters
Obsessions can drive great achievements in science or art, but they often lead to mental health struggles. They can damage relationships and prevent a person from living a balanced life.
Easy to mix up
It is often confused with a hobby or a passion, but an obsession is much harder to control and often causes distress.