"Sexual inversion" is a term used by sexologists, primarily in the late 19th and early 20th century, to refer to homosexuality. Sexual inversion was believed to be an inborn reversal of gender traits: male inverts were, to a greater or lesser degree, inclined to traditionally female pursuits and dress and vice versa. The sexologist Richard Freiherr von Krafft-Ebing described female sexual inversion as "the masculine soul, heaving in the female bosom". In its emphasis on gender role reversal, the theory of sexual inversion resembles transgender, which did not yet exist as a separate concept at the time.
Initially confined to medical texts, the concept of sexual inversion was given wide currency by Radclyffe Hall's 1928 lesbian novel The Well of Loneliness, which was written in part to popularize the sexologists' views. Published with a foreword by the sexologist Havelock Ellis, it consistently used the term "invert" to refer to its protagonist, who bore a strong resemblance to one of Krafft-Ebing's case studies.