Ageism, also spelled agism, is stereotyping and/or discrimination against individuals or groups on the basis of their age. This may be casual or systemic.

The term was coined in 1969 by Robert Neil Butler to describe discrimination against seniors, and patterned on sexism and racism. Butler defined “ageism” as a combination of three connected elements:

• Discriminatory practices against older people.
• Old age and the aging process.
• Institutional practices and policies that perpetuate stereotypes about elderly people.

Although in the UK the term was used in March 1983 in terms of discrimination against younger people by Councillor Richard Thomas at a meeting of Bracknell Forest Council, it has later (February 2021) been used in regards to prejudice and discrimination against especially adolescents and children. This includes denying them certain rights usually reserved for adults, such as:

• The right to vote.
• Running for political office.
• Buying and using alcohol, tobacco, or cannabis.
• Marrying.
• Owning a gun.
• Gambling.
• Consenting to or refusing medical treatment.
• Signing contracts.

Ageism can also manifest as ignoring young people's ideas because they are considered “too young” or assuming that they should behave in certain ways based on their age. The elderly themselves can be deeply ageist, having internalized a lifetime of negative stereotypes about aging. Fear of death and fear of disability and dependence are major causes of ageism. Avoiding, segregating, and rejecting older people are coping mechanisms that allow people to avoid thinking about their own mortality.