Sex refers to the aspects of a person's biology which are sexually dimorphic. This includes chromosomes, hormone levels, genitalia and secondary sex characteristics such as breast size, hip to waist ratio, body hair and voice pitch. These characteristics are sometimes viewed as being linked to a person's gender, although in reality sex and gender are separate concepts.
Although each person's sex characteristics are slightly different, sex is often grouped broadly into two categories: male and female refereed to as binary or dyadic sexes. Sex is typically assigned at birth based on the appearance of the genitals, and may not reflect gender identity later in life. People whose sex cannot be categorized clearly as male or female are referred to as intersex, and they may undergo medical treatment (often non-consensually in childhood) so that their biology more closely matches their assigned sex.
Unlike gender expression, sex characteristics can typically only be consciously altered through medical treatment, such as surgery or hormone replacement therapy. Transgender people often undergo such treatments as part of their transition. Cisgender people may also alter their sex characteristics through medical treatments, such as by taking hormonal birth control or a mastectomy (breast removal) to treat breast cancer. Sex characteristics which do not align with gender identity are often a focus of bodily dysphoria.
Referring to people via their sex is typically seen as disrespectful, as gender identity (man, woman etc.) is far more relevant in social interactions. In cases where sex characteristics are relevant, such as a medical setting, it is more important to consider their individual biology and history than which broad categorization they fall under. Significant variation exists among any assigned sex, and any broad statements on differences between the sexes are true only on average, with little effect on individuals.