Heterosexism is a system of attitudes, bias, and discrimination in favor of heterosexuality and heterosexual relationships. According to Elizabeth Cramer, it can include the belief that all people are or should be heterosexual and that heterosexual relationships are the only norm and therefore superior.

Although heterosexism is defined in the online editions of the American Heritage Dictionary of the English Language and the Merriam-Webster Collegiate Dictionary as anti-gay discrimination or prejudice "by heterosexual people" and "by heterosexuals", respectively, people of any sexual orientation can hold such attitudes and bias, and can form a part of internalised hatred of one's sexual orientation.

Heterosexism as discrimination ranks gay men, lesbians, bisexuals and other sexual minorities as second-class citizens with regard to various legal and civil rights, economic opportunities, and social equality in many of the world's jurisdictions and societies. It is often related to homophobia.

While the Merriam-Webster Collegiate Dictionary notes the first use of the term heterosexism as having occurred in 1972, the term was first published in 1971 by gay rights activist, Craig Rodwell.

Similar terms include "heterocentrism" and "heterosexualism". Although the well-established term heterosexism is often explained as a coinage modeled on sexism, the derivation of its meaning points more to (1.) heterosex(ual) + -ism than (2.) hetero- + sexism.

Given this lack of semantic transparency, researchers, outreach workers, critical theorists and LGBTQ activists have proposed and use terms such as institutionalized homophobia, state(-sponsored) homophobia, sexual prejudice, anti-gay bigotry, straight privilege, The Straight Mind (a collection of essays by French writer Monique Wittig), heterosexual bias, compulsory heterosexuality or the much lesser known terms heterocentrism, homonegativity, and from gender theory and queer theory, heteronormativity. However, not all of these descriptors are synonymous with heterosexism.

Homophobia, a form of heterosexism, refers both to "unreasoning fear of or antipathy towards homosexuals and homosexuality" and to "behavior based on such a feeling". Heterosexism, however, more broadly denotes the "system of ideological thought that makes heterosexuality the sole norm to follow for sexual practices". As a bias favoring heterosexuals and heterosexuality, heterosexism has been described as being "encoded into and characteristic of the major social, cultural, and economic institutions of our society" and stems from the essentialist cultural notion that maleness-masculinity and femaleness-femininity are complementary.

Psychology professor Gregory M. Herek states that "[Heterosexism] operates through a dual process of invisibility and attack. Homosexuality usually remains culturally invisible; when people who engage in homosexual behavior or who are identified as homosexual become visible, they are subject to attack by society." Furthermore, in interviews with perpetrators of anti-gay violence, forensic psychologist Karen Franklin believes that "heterosexism is not just a personal value system, [rather] it is a tool in the maintenance of gender dichotomy." She continues by saying that "assaults on homosexuals and other individuals who deviate from sex role norms are viewed as a learned form of social control of deviance rather than a defensive response to personal threat."