Sexual orientation describes an enduring pattern of attraction—emotional, romantic, sexual, or some combination of these—to the opposite sex, the same sex, or both sexes, and the genders that accompany them. These attractions are generally subsumed under heterosexuality, homosexuality, and bisexuality. Asexuality (the lack of romantic or sexual attraction to others) is sometimes identified as the fourth category. These categories are aspects of the more nuanced nature of sexual identity. For example, people may use other labels or none at all. According to the American Psychological Association, sexual orientation also refers to a person's sense of "personal and social identity based on those attractions, behaviors expressing them, and membership in a community of others who share them."

The term sexual preference largely overlaps with sexual orientation, but is distinguished in psychological research. A person who identifies as bisexual, for example, may sexually prefer one sex over the other. "Sexual preference" may also suggest a degree of voluntary choice. This is disputed in terms of sexual formation, as consensus among scholars is that sexual orientation is not a choice. There is no simple, single cause for sexual orientation that has been conclusively demonstrated, but research suggests that it is by a combination of genetic, hormonal, and environmental influences, with biological factors involving a complex interplay of genetic factors and the early uterine environment. Research over several decades has demonstrated that sexual orientation ranges along a continuum, from exclusive attraction to the opposite sex to exclusive attraction to the same sex.