Heterophile is an alternate term to heterosexual and once competed with it for currency, but only did so successfully in Scandinavia. It is a parallel concept to homophile.
In Scandinavia and Finland, the word heterofil is a gender-neutral term, like "heterosexual" in Canada and the United States.
Heterophile antibodies are antibodies induced by external antigens (heterophile antigens) that cross-react with self-antigens. For example, antibodies against group A streptococcal cell walls can also react with (and thus damage) human heart tissues. These are considered heterophile antibodies.
In clinical diagnosis, the heterophile antibody test specifically refers to a rapid test for antibodies produced against the Epstein-Barr virus (EBV), the causative agent of infectious mononucleosis.