Heterosexuality refers to the condition of being sexually attracted exclusively to members of the opposite sex and/or gender. Often seen as the opposite of homosexuality, it is also distinct from bisexuality, since a bisexual person is also attracted to members of their own sex and gender (and, often, to people of several non-binary genders), and asexuality, since an asexual person is not attracted to anyone. Heterosexuality can be considered one end of a spectrum which describes an individual's sexual identity as relates to gender, since many people are attracted to men and women to varying degrees. Common scales used are the Kinsey Scale and the Klein Sexual Orientation Grid. The majority of the population is heterosexual, leading to non-heterosexual identities being frequently marginalized or misunderstood.
The term is typically used for people who fall clearly at one end or the other of the gender spectrum. A sexual relationship between a man and a woman would be considered heterosexual (whether the man and women are cis or trans), but a relationship between a genderqueer person and a man or a woman usually would not be, even though they are of different genders. Gender expression is usually not considered (e.g. a relationship between a woman and a drag queen would be considered heterosexual).