Claustrophobic can be used to describe someone with a fear of small, enclosed spaces. If you're claustrophobic, there is almost nothing scarier than the prospect of being trapped in an elevator for a long time.
The Latin phobia means fear. In this case, it's paired with claudere, which means "to close;" claustrophobic describes a fear of being in a small, closed-off spaces. You can use this word to talk about someone with a psychiatric condition or with a milder affliction, even just a dislike for tiny or enclosed areas. You can also use claustrophobic to describe a place that makes you feel panicked, like the claustrophobic room at the library where your manga club meets every week.