This article is about biological sexes — male, female, etc. For alternate uses, such as gender or human sexual behaviour, see Sex (disambiguation)
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Sex refers to the male and female duality of biology and reproduction, a process in biological DNA that dates back 4.6 - 3.5 billion years. DNA links back in an unbroken series of sexual reproduction taking forward information to present day. The somewhat similar term gender has more to do with identity than biology. The concept is confined to organisms that reproduce sexually.

The female sex is definitely defined as the one which produces the larger gamete (i.e., reproductive cell) and which typically bears the offspring. The category of sex reflects the biological reproductive function, rather than sexuality or other behaviors. In some animals, sex may be assigned to specific structures rather than the entire organism as some species, such as earthworms, are normally hermaphroditic
In humans, sex is conventionally perceived as a dichotomous state or identity for most biological and social purposes, such that a person can only be female or male. However, when the criteria generally used to define femaleness and maleness are examined more closely, it becomes apparent that the assignment or determination of 'sex' occurs at multiple levels. Environmental, biological, social, psychological and other factors are all believed to have some role in this process, and the complex interaction of these factors is expressed in the diversity of biological and psychosocial 'states' or levels found amongst the human population. A significant fraction of the human population simply does not correspond exclusively to either 'female' or 'male' with regard to every level of definition expressed in the following table. This discordance is discussed in more detail below.

This table outlines the major levels at which society currently recognizes a difference between human females and males. Some criteria are dichotomous and some, such as body size, exhibit sexual dimorphism (i.e. characteristics which are statistically more likely to be found in one sex than the other). Some of the levels are more amenable to scientific study or measurement than others; some are "imputed" or assigned to individuals by the society of which they are members (e.g. whether human males must wear trousers is a result of social norms); and some seem to be generated within each individual as a subjective identity or drive.

"Primary" sexual characteristics are typically present at birth and directly involved in reproduction. "Secondary" sexual characteristics typically develop later in life (usually during puberty) and are not directly involved in reproduction.