What is the meaning of Third_gender?

A category (a gender), present in societies which recognize four or more genders, which is neither cis man/cis boy nor cis woman/cis girl; often, such societies consider trans man/trans boy/transmasculine to constitute a fourth gender and trans woman/trans girl/transfeminine to constitute a third gender.

A trans woman/trans girl/transfeminine

Source: wiktionary.org