Heterosexuality is romantic attraction, sexual attraction, or sexual behavior between people of the opposite sex or gender. As a sexual orientation, heterosexuality is "an enduring pattern of emotional, romantic, and/or sexual attractions" to people of the opposite sex. It "also refers to a person's sense of identity based on those attractions, related behaviors, and membership in a community of others who share those attractions." Someone who is heterosexual is commonly referred to as straight.

Along with bisexuality and homosexuality, heterosexuality is one of the three main categories of sexual orientation within the heterosexual–homosexual continuum. Across cultures, most people are heterosexual, and heterosexual activity is by far the most common type of sexual activity. Heterosexuality has mostly been viewed as the normative and most socially dominant form of sexual orientation.

Scientists do not know the exact cause of sexual orientation, but they theorize that it is caused by a complex interplay of genetic, hormonal, and environmental influences, and do not view it as a choice. Although no single theory on the cause of sexual orientation has yet gained widespread support, scientists favor biologically based theories. There is considerably more evidence supporting nonsocial, biological causes of sexual orientation than social ones, especially for males.

The term heterosexual or heterosexuality is usually applied to humans, but heterosexual behavior is observed in all other mammals and in other animals, as it is necessary for sexual reproduction.

Hetero- comes from the Greek word ἕτερος [héteros], meaning "other party" or "another", used in science as a prefix meaning "different"; and the Latin word for sex (that is, characteristic sex or sexual differentiation).

The current use of the term heterosexual has its roots in the broader 19th century tradition of personality taxonomy. The term heterosexual was coined alongside the word homosexual by Karl Maria Kertbeny in 1869. The terms were not in current use during the late nineteenth century, but were reintroduced by Richard von Krafft-Ebing and Albert Moll around 1890. The noun came into wider use from the early 1920s, but did not enter common use until the 1960s. The colloquial shortening "hetero" is attested from 1933. The abstract noun "heterosexuality" is first recorded in 1900. The word "heterosexual" was listed in Merriam-Webster's New International Dictionary in 1923 as a medical term for "morbid sexual passion for one of the opposite sex"; however, in 1934 in their Second Edition Unabridged it is defined as a "manifestation of sexual passion for one of the opposite sex; normal sexuality".

Hyponyms of heterosexual include heteroflexible.

The word can be informally shortened to "hetero". The term straight originated as a mid-20th century gay slang term for heterosexuals, ultimately coming from the phrase "to go straight" (as in "straight and narrow"), or stop engaging in homosexual sex. One of the first uses of the word in this way was in 1941 by author G. W. Henry. Henry's book concerned conversations with homosexual males and used this term in connection with people who are identified as ex-gays. It is now simply a colloquial term for "heterosexual", having changed in primary meaning over time. Some object to usage of the term straight because it implies that non-heterosexual people are crooked.