Prejudice is making a judgment or assumption about someone or something before having enough knowledge to be able to do so with guaranteed accuracy, or "judging a book by its cover". The word prejudice is most often used to refer to preconceived judgments toward people or a person because of race, social class

Social class

Social classes are economic or cultural arrangements of groups in society. Class is an essential object of analysis for sociologists, political scientists, economists, anthropologists and social historians. In the social sciences, social class is often discussed in terms of 'social stratification'...

, ethnicity, age

Ageing

Ageing or aging is the accumulation of changes in a person over time. Ageing in humans refers to a multidimensional process of physical, psychological, and social change. Some dimensions of ageing grow and expand over time, while others decline...

, disability

Disability

A disability may be physical, cognitive, mental, sensory, emotional, developmental or some combination of these.Many people would rather be referred to as a person with a disability instead of handicapped...

, obesity

Obesity

Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy and/or increased health problems...

, religion

Religion

Religion is a collection of cultural systems, belief systems, and worldviews that establishes symbols that relate humanity to spirituality and, sometimes, to moral values. Many religions have narratives, symbols, traditions and sacred histories that are intended to give meaning to life or to...

, sexual orientation

Sexual orientation

Sexual orientation describes a pattern of emotional, romantic, or sexual attractions to the opposite sex, the same sex, both, or neither, and the genders that accompany them. By the convention of organized researchers, these attractions are subsumed under heterosexuality, homosexuality,...

, or other personal characteristics. It also means beliefs without knowledge of the facts and may include "any unreasonable attitude that is unusually resistant to rational influence."
The first psychological research conducted on prejudice occurred in the 1920s.