Prejudice is prejudgment, or forming an opinion before becoming aware of the relevant facts of a case. To speak negatively against someone because of their gender, religion, sexuality, race/ethnicity, language or nationality.

Intolerance of or dislike for people of a specific race, religion, etc.

The word is often used to refer to preconceived, usually unfavorable, judgments toward people or a person because of gender, political opinion, social class, age, disability, religion, sexuality, race/ethnicity, language, nationality, or other personal characteristics. 

In this case, it refers to a negative evaluation of another person based on their perceived group membership. 

Prejudice can also refer to unfounded beliefs and may include “any unreasonable attitude that is unusually resistant to rational influence”.

Gordon Allport defined prejudice as a “feeling, favorable or unfavorable, toward a person or thing, prior to, or not based on, actual experience”.
When the opinions deriving from prejudice reaches the state of influencing policy making in politics and culture in a society, this is when prejudice becomes upgraded to racism.

Historical approaches:
The first psychological research conducted on prejudice occurred in the 1920s. This research attempted to prove white supremacy. One article from 1925 reviewing 73 studies on race concluded that the studies seemed “to indicate the mental superiority of the white race”.

These studies, along with other research, led many psychologists to view prejudice as a natural response to inferior races.