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.

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.