Sexual orientation refers to individual's propensity for romantic and erotic attachments.
Heterosexuality refers to attaching to a partner with different anatomy; homosexuality refers to a same-sex partner; bisexuality refers to attaching to both men and women.
Other definition:
Sexual orientation refers to the degree to which a person is erotically attracted to members of the same or opposite sex.
Other /More definition:
Sexual orientation refers to one's preference for partners of the same or opposite sex with respect to attraction and sexual desire.