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1892, in C.G. Craddock's translation of Krafft-Ebbing's "Psychopathia Sexualis," a hybrid; see hetero- + sexual. The noun is recorded from 1920, but not in common use until 1960s. Colloquial shortening hetero is attested from 1933.
heterosexual het·er·o·sex·u·al (hět'ə-rō-sěk'shōō-əl)adj. Sexually oriented to persons of the opposite sex. n. A heterosexual person.