Preconceived means formed or decided before you have actual evidence or experience. A preconceived idea is one you've already made up your mind about before really examining it.

When you have preconceived notions about something, you've jumped to conclusions. Maybe you decided a book would be boring before reading the first page, or assumed a new student would be unfriendly before meeting them. These judgments happen in your mind first, often based on limited information, assumptions, or what others have told you.

Scientists work hard to avoid preconceived ideas about their experiments because those ideas can make them see only what they expect to see rather than what's actually happening. A good detective can't let preconceived theories about a case stop them from following the real evidence.

Having some expectations is natural, but preconceived ideas become problems when they're so fixed that you ignore evidence that contradicts them. The fairest approach is staying curious and open, letting reality speak for itself rather than deciding everything in advance.