This is from the Stonewall website....
Gender dysphoria
Used to describe when a person experiences discomfort or distress because there is a mismatch between their sex assigned at birth and their gender identity.
This is also the clinical diagnosis for someone who doesn’t feel comfortable with the sex they were assigned at birth.
Cisgender or Cis
Someone whose gender identity is the same as the sex they were assigned at birth. Non-trans is also used by some people.
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What you see here is the central position of gender identity, around which everything else orbits. Trans activists believe we all have a gender identity, you, me, everyone, and it is our gender identity that defines us. For some our gender identity matches our "sex assigned at birth", we are cisgender. For those whose gender identity does not match their "sex assigned at birth", they can be any one of a multitude of various genders or even "gender fluid". But wherever folk lie on this spectrum, that is who they are. Hence the drive to get rid of procedures and medical professionals and replace them with self ID, because Self ID is simply an affirmation of ones true self.
If you believe all this, then your sex, your biological sex, is immaterial, hence the rallying cry "Trans Women Are Women! Trans men are men!" And this is the new reality because it is ones gender identity that defines you, over and above all else.
So if you're born in a male body, but your gender identity is that of a woman (whatever that is), then that makes you a woman, hence trans women are women. And if you're born in a female body, but your gender identity is that of a man (again, whatever that is), then that makes you a man, hence trans men are men.