The term eonism was coined by Havelock Ellis to describe similar cases of transgender behavior; it is rarely used now.

In 1920 he coined the term eonism, which he derived from the name of a historical figure, Chevalier d'Eon.

Today, his notion of eonism has been abandoned in favor of autogynephilia, a term with a simpler definition coined by Ray Blanchard.

As in the Freudian tradition, Ellis postulated that a "too close attachment to the mother" may encourage eonism, but also considered that it "probably invokes some defective endocrine balance".

Ellis found eonism to be "a remarkably common anomaly", and "next in frequency to homosexuality among sexual deviations", and categorized it as "among the transitional or intermediate forms of sexuality."