Gender is the range of characteristics pertaining to, and differentiating between, masculinity and femininity. Depending on the context, these characteristics may include biological sex (i.e. the state of being male, female or intersex), sex-based social structures (including gender roles and other social roles), or gender identity.[1][2][3][4]

Sexologist John Money introduced the terminological distinction between biological sex and gender as a role in 1955. Before his work, it was uncommon to use the word gender to refer to anything but grammatical categories.[1][2] However, Money's meaning of the word did not become widespread until the 1970s, when feminist theory embraced the concept of a distinction between biological sex and the social construct of gender. Today, the distinction is strictly followed in some contexts, especially the social sciences[5][6] and documents written by the World Health Organization (WHO).[4] However, in many other contexts, including some areas of social sciences, gender includes sex or replaces it.[1][2] Although this change in the meaning of gender can be traced to the 1980s, a small acceleration of the process in the scientific literature was observed in 1993 when the Food and Drug Administration (FDA) started to use gender instead of sex.[7] In 2011, the FDA reversed its position and began using sex as the biological classification and gender as "a person's self representation as male or female, or how that person is responded to by social institutions based on the individual's gender presentation."[8] In non-human animal research, gender is also commonly used to refer to the physiology of the animals.[2]

In the English literature, the trichotomy between biological sex, psychological gender, and social sex role first appeared in a feminist paper on transsexualism in 1978.[2][9] Some cultures have specific gender-related social roles that can be considered distinct from male and female, such as the hijra of India and Pakistan.

The social sciences have a branch devoted to gender studies. Other sciences, such as sexology and neuroscience, are also interested in the subject. While the social sciences