A monosexual is a person with a sexual preference for a single sex.
Hungarian journalist Karl Maria Kertbeny coined the term in the late 1860s. By his definition, a monosexual is a person that masturbates or who has a sexual interest only in themselves. However, that use of the term is now obsolete.
Recently, the term monosexual is understood as an alternative to bisexual, a term describing a person with a sexual interest in two sexes: male and female. The term is typically used in the academic world.