https://www.ons.gov.uk/economy/environmentalaccounts/articles/whatisthedifferencebetweensexandgender/2019-02-21
2.Definitions and differences
The UK government defines sex as:
referring to the biological aspects of an individual as determined by their anatomy, which is produced by their chromosomes, hormones and their interactions
generally male or female
something that is assigned at birth
The UK government defines gender as:
a social construction relating to behaviours and attributes based on labels of masculinity and femininity; gender identity is a personal, internal perception of oneself and so the gender category someone identifies with may not match the sex they were assigned at birth
where an individual may see themselves as a man, a woman, as having no gender, or as having a non-binary gender – where people identify as somewhere on a spectrum between man and woman
The World Health Organisation regional office for Europe describes sex as characteristics that are biologically defined, whereas gender is based on socially constructed features. They recognise that there are variations in how people experience gender based upon self-perception and expression, and how they behave.
Essentially, nearly all people are born with physical characteristics that are labelled male or female. In 1964, Robert Stoller coined the term gender identity, which refers to an individual’s personal concept about their gender and how they feel inside. It is a deeply held internal sense of self and is typically self-identified. Gender identity differs from sexual identity and is not related to an individual’s sexual orientation (for more information, see the Terminology page of the Gender Identity Research and Education Society). As such, the gender category with which a person identifies may not match the sex they were assigned at birth.
Gender is increasingly understood as not binary but on a spectrum. Growing numbers of people are identifying as somewhere along a continuum between man and woman, or as non-gendered (neither man nor woman) (see Gender Spectrum). Therefore, they often have their own words to describe themselves rather than using pre-defined categories of male and female (for more information, see Gender Identity Workshop, Summary of Discussions). While more people are identifying as non-binary, this is not a new concept and has existed for many years across different cultures around the world.