To obsess means to think about something so constantly and intensely that it's hard to focus on anything else. When you obsess over something, your mind keeps returning to it like a song stuck on repeat. You might obsess about an upcoming performance, replaying every possible mistake in your head, or obsess over whether your friend is mad at you, analyzing every word of your last conversation.

Obsessing goes beyond normal thinking or worrying. If you're excited about your birthday party, you're looking forward to it. If you're obsessing about your birthday party, you can barely concentrate on schoolwork because you're constantly thinking about who's coming, what games you'll play, and whether everything will go perfectly.

The word can describe both positive and negative fixations. Someone might obsess over collecting every card in a series, or obsess about getting a perfect score on a test. But obsessing usually means the thoughts have become excessive and overwhelming, taking up more mental energy than they deserve.

When someone is obsessed with something, they've crossed the line from healthy interest into an all-consuming preoccupation. An obsession is the thing someone obsesses about. The key difference between caring deeply about something and obsessing over it is whether you can still think about other things, or if that one concern has taken over your mind completely.