Ageism is a type of discrimination based on one's age, generally used to refer to age-based discrimination against elderly people. The term was coined in 1969 by Robert Neil Butler to describe this discrimination, building on the terminology of sexism and racism. Butler defined ageism as a combination of three connected elements: negative attitudes towards old age and the ageing process, discriminatory practices against older people, and institutional practices and policies that perpetuate stereotypes about elderly people.

The term "ageism" is also used to describe the oppression of younger people by older people. An example is a 1976 pamphlet published by Youth Liberation of Ann Arbor, Michigan. In the UK, at a meeting of the Bracknell Forest Council in June 1983, councillor Richard Thomas pointed out that age discrimination works against younger and older people. This includes the practice of denying younger people certain rights and privileges usually reserved for adults. These include the right to vote, run for political office, refuse medical treatment, and sign contracts. This definition of ageism can also include ignoring the ideas and contributions of adolescents and children because they are considered "too young" or dismissing their behavior as caused by their age. Ageism against the young also includes penalties, burdens, or requirements imposed exclusively (or to a greater degree) on young people than on older people, such as age-based military conscription.

Older people themselves can be ageist, having internalized a lifetime of negative stereotypes about aging. Ageism is often connected to fears of death and disability- with avoiding, segregating, and rejecting older people functioning as a coping mechanism to avoid these concepts. There is a large overlap between ageism and ableism, discrimination based on disability.

Ageism in common linguistic and age studies usually refers to negative discriminatory practices against old people, people in their middle years, teenagers, and children. There are several forms of age-related bias. Adultism is a predisposition towards adults, which is seen as bias against children, youth, and all young people who are not addressed or viewed as adults. This includes political candidacies, jobs, and cultural settings where the supposed greater vitality and physical beauty of youth are less appreciated than the supposed greater moral and intellectual rigor of adulthood. Adultcentrism is the exaggerated egocentrism of adults. Adultocracy is the social convention which defines "maturity" and "immaturity", placing adults in a dominant position over young people, both theoretically and practically. Gerontocracy is a form of oligarchical rule in which an entity is ruled by leaders significantly older than most of the adult population. Chronocentrism is the belief that a certain state of humanity is superior to all previous and/or future times.

In 2009, Iversen, Larsen, and Solem introduced a new definition of ageism based on a conceptual analysis of the term. This definition serves as a foundation for future research on ageism. It also provides an approach to understanding the complexities of ageism. They define ageism as "negative or positive stereotypes, prejudice, and/or discrimination against (or to the advantage of) elderly people based on their chronological age or the perception of them as being 'old' or 'elderly.' Ageism can be implicit or explicit and may manifest on micro, meso, or macro levels" (Iversen, Larsen, and Solem, 2009).

Other conditions of fear or aversion associated with age groups have their own names. Paedophobia is a fear of infants and children, ephebiphobia is the fear of youth and is also referred to as an irrational fear of adolescents or a prejudice against teenagers, and gerontophobia is a fear of elderly people.

Implicit ageism refers to thoughts, feelings, and judgments that operate without conscious awareness and are automatically produced in everyday life. These may be a mixture of positive and negative thoughts and feelings, but gerontologist Becca Levy reports that they "tend to be mostly negative".

Stereotyping is a tool of cognition that involves categorizing into groups and attributing characteristics to these groups. Stereotypes are necessary for processing huge volumes of information, which would otherwise overload a person. They are generally accurate descriptors of group characteristics, though some stereotypes are inaccurate. However, they can cause harm when the content of the stereotype is incorrect concerning most of the group or where a stereotype held too heavily overrides evidence which shows that an individual does not conform to it. For example, age-based stereotypes may cause one to draw very different conclusions when one sees an older and a younger adult with, for example, back pain or a limp. One might assume that following an accident, the younger person's condition is temporary and treatable, while the older person's condition is chronic and less susceptible to intervention. While this might generally be true, many elderly individuals recover quickly from accidents, and conversely, very young people—such as infants, toddlers, and small children—can become permanently disabled under similar circumstances. This assumption may not matter in casual encounters, like passing someone on the street. However, if held by healthcare professionals or managers responsible for occupational health, it could lead to inappropriate actions and age-related discrimination.^{[citation needed]}