Preoccupation and Thing

Preoccupation noun - Something about which one is constantly thinking or concerned.

Thing and preoccupation are semantically related in bee in one's bonnet topic. In some cases you can use "Thing" instead a noun "Preoccupation". informal substitute

Thing noun - A member of the human race.

Preoccupation and thing are semantically related in idea topic. Sometimes you can use "Preoccupation" instead a noun "Thing", if it concerns topics such as concern.