The gender binary, also referred to as gender binarism (sometimes shortened to just binarism), is the classification of sex and gender into two distinct, opposite and disconnected forms of masculine and feminine. It is one general type of a gender system. As one of the core principles of genderism, it can describe a social boundary that discourages people from crossing or mixing gender roles, or from identifying with third (or more) forms of gender expression altogether. It can also represent some of the prejudices which stigmatize intersex and transgender people, especially those that are genderqueer-identified — individuals who may not always fit neatly into the gender binary.
The term describes the system in which a society splits people into male and female gender roles, gender identities and attributes. Gender role is one aspect of a gender binary. Many of known societies have used the gender binary to divide and organize people, though the ways this happens differ among societies. A universal aspect of the gender binaries is that women give birth. Gender binaries exist as a means of bringing order, though some, such as Riki Wilchins in GenderQueer: Voices from Beyond the Sexual Binary, argue that gender binaries divide and polarize society. Certain notable religions are often used as authorities for the justification and description. Islam, for example, teaches that mothers are the primary care givers to their children and Catholics only allow males to serve as their priests.
Exceptions have widely existed to the gender binary in the form of specific transgender identities. Besides the biological identification of intersex individuals, elements of the both or neither sexes have been taken by people biologically female and male such as Two-Spirited Native Americans and hijra of India. In the contemporary West, transgender people break the gender binary in the form of genderqueer. Transsexuals have a unique place in relation to the gender binary because in many cases their gender expression transitions from one side of the gender binary to the other but still conforms to the gender binary itself. Other forms of gender identity which may fall outside of the gender binary include bigender, androgyne, and genderfuck.