It is the opposite of a hypothesis, which evokes one eventuality among several, in a methodical approach. It remains a prejudice as long as the person making it does not question it.
Prejudice can be both positive and negative. It can also have an effect that seems positive (cohesion within a group or society), but is in fact negative (exclusion, discrimination, denigration of all others, xenophobia, racism) in a multicultural society.
The word “prejudice” generally refers to a belief that we ourselves consider to be unfounded, and which we attribute to others; “prejudices” are the “irrational beliefs of others“. To qualify a prejudice to which we ourselves adhere, we generally use alternative concepts such as “self-evident“, “fundamental truth“, “acquired principle“, etc.
Source: Wikipedia, article Préjugé (in French).