The Austrian philosopher Otto Weininger used the term "transsexuality" to describe the opposite-sex parts of a human being, which he saw embodied in the figure of Parsifal by Richard Wagner. Transsexual meant to him everything non-male, including books, politics, science, and art, which would be antithetical to the sexual - for Weininger synonymous with the phallus. Weininger argued in his book Gender and Character that only the sexual, not the transsexual, was attractive to women. Weininger's definitions, however, have (apart from a few errors) almost nothing to do with the modern understanding of transsexuality.
In 1910, German physician and sex researcher Magnus Hirschfeld coined the term transvestite to describe people who occasionally, regularly, or constantly dress as members of the opposite sex. Hirschfeld's original definition of "transvestite," however, was much broader than the modern definition, despite the outward focus of dress; it also included transsexual phenomena, similar to today's term "transgender," but focusing on internal gender identity. In 1923 - in the last edition of his Jahrbuch für sexuelle Zwischenstufen (Yearbook for Sexual Intermediates) - he used the term "psychic transsexualism" for people who did not simply dress in the opposite sex but belonged mentally to one sex but physically to the other, a gender variation that he regarded as a precursor to hermaphroditism.
For a time, David O. Cauldwell, who had taken up the word in his 1949 article Psychopathia transexualis, was mistakenly considered the originator of the term. Harry Benjamin, who knew Hirschfeld, his publications, and his Institute of Sexology, took up the term again in 1953 in his article Transvestism and Transsexualism in connection with the case of Christine Jorgensen, and established it in sexual medicine in 1966 with his book The Transsexual Phenomenon. In the works of Cauldwell and Benjamin, the term transsexualism was already used in its current meaning.
In the 1990s, the term transsexualism was removed from the Diagnostic and Statistical Manual of Mental Disorders, the DSM-IV, and replaced by the term gender identity disorder. In contrast, the World Health Organization's International Classification of Diseases, ICD-10 still used the terms transsexualism and gender identity disorder interchangeably. Transsexualism is found under Class F (Mental and behavioural disorders) as subheading F64.0 in paragraph F64 (Gender identity disorders) of Chapter F6 (Personality and behavioural disorders).
In the new draft of the ICD-11 classification, only gender incongruence is mentioned, which means non-conformity of the gender characteristics of the body and/or the social role with the identity gender.