What is Gender?

Legal Definition
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 an intersex variation which may complicate sex assignment), sex-based social structures (including gender roles and other social roles), or gender identity. Some cultures have specific gender roles that can be considered distinct from male and female, such as the hijra (chhaka) of India and Pakistan.

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. 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 and documents written by the World Health Organization (WHO).

In other contexts, including some areas of social sciences, gender includes sex or replaces it. For instance, in non-human animal research, gender is commonly used to refer to the biological sex of the animals. This change in the meaning of gender can be traced to the 1980s. In 1993, the USA's Food and Drug Administration (FDA) started to use gender instead of sex. Later, 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."

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 sometimes approach gender as a social construct, and gender studies particularly do, research in the natural sciences investigates whether biological differences in males and females influence the development of gender in humans; both inform debate about how far biological differences influence the formation of gender identity. In the English literature, there is also a trichotomy between biological sex, psychological gender, and social gender role. This framework first appeared in a feminist paper on transsexualism in 1978.
-- Wikipedia
Legal Definition
Defined difference between men and women based on culturally and socially constructed mores, politics, and affairs. Time and location give rise to a variety of local definitions. Contrasts to what is defined as the biological sex of a living creature.
Legal Definition
That which designates the sexes.

2. As a general rule, when the masculine is used it includes the feminine, as, man (q. v.) sometimes includes women. This is the general rule, unless a contrary intention appears. But in penal statutes, which must be construed strictly, when the masculine is used and not the feminine, the latter is not in general included. 3 C. & P. 225. An instance to the contrary, however, may be found in the construction, 25 Ed. III, st. 5, c. 2, ยง1, which declares it to be high treason, "When a man doth compass or imagine the death of our lord the king," &c. These words, "our lord the king," have been construed to include a queen regnant. 2 Inst. 7, 8, 9; H. P. C. 12; 1 Hawk. P. C. c. 17; Bac. Ab. Treason, D.

3. Pothier says that the masculine often includes the feminine, but the feminine never includes the masculine; that according to this rule if a man were to bequeath to another all his horses, his mares would pass by the legacy; but if he were to give all his mares, the horses would not be included. Poth. Introd. au titre 16, des Testaments et Donations Testamentaires, n. 170; 3 Brev. R. 9. In the Louisiana code in the French language, it is provided that the word fils, sons, comprehends filles, daughters. Art. 3522, n. 1. Vide Ayl. Pand. 57; 4 Car. & Payne, 216; S. C. 19 Engl. Com. Law R. 351; Barr. on the Stat. 216, note; Feme; Feme covert; Feminine; Male; Man; Sex; Women; Worthiest of blood.
-- Bouviers Law Dictionary