Definition 1 of prejudice in a lazy google search is "preconceived opinion that is not based on reason or actual experience." Certainly that figures into most reasoning, considering that perfect information is impossible.
That definition seems to cover everything you've learned a textbook, video, lecture, another person, or in any other indirect way, and which you didn't have an opportunity to think through yet.
Which is... most of the thing people know? Including, ironically, this very definition, which I learned about from a HN comment that quoted a Google search result...