Gender identity is your psychological sense of self. Who you, in your head, know yourself to be, based on how much you align (or don’t align) with what you understand to be the options for gender.
Generally, gender identity is assumed at birth to match the anatomical sex that you are assigned. This is where the concept of being ‘trans’ comes in. It is an umbrella term saying that you don’t feel like the gender you were assigned, whether this is because you feel like the other, neither or a combination. As a term ‘trans’ has often replaced ‘transgender’ which is often associated with binary changes between man/woman and therefore does not reflect the whole trans community.
Though the existence of trans people is often treated as a debate in the media, from a scientific perspective this is simply not the case. For example, the royal college of psychiatrists says in its 2018 position statement that “The College supports psychiatrists in fully exploring their patient’s gender identity (involving their families where appropriate) in a non-judgemental, supportive and ethical manner.” And this view is reflected across the medical community.
Those individuals that fall outside the ‘traditional’ idea of transgender (i.e., trans men and women) are often termed non-binary. That is, they do not feel that either man or woman reflects their identity and may feel like a combination of both or neither. Such individuals often face even more discrimination with society generally not accepting of those outside the binary definitions.
The very idea of trans individuals raises questions about what gender is, try to define a man or woman without using physical or biological characteristics and you’ll often find yourself falling into stereotypes! But each person should be able to explore this for themselves and it is important we respect the choices that people make.
Sources: https://www.rcpsych.ac.uk/pdf/PS02_18.pdf