A thought or concern that takes up most of your attention because you keep thinking about it.
From Latin 'praeoccupatio,' from 'prae' (before) and 'occupare' (to seize), literally meaning 'seized beforehand.' The word evolved to mean mentally seized by a thought.
Neuroscientists call this 'cognitive capture'—when your brain gets preoccupied, it actually reduces activity in areas handling new information, which is why you can't focus on homework when you're worried!
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