What is ‘transgender’?

I’ve been asked by a friend to ‘explain’ transgender for when she has to stand up against transphobic assholes online. It’s testament to how complex, and diverse, trans* experience and identity is that I as a trans* and gender activist, and a trans* person myself, took two days to formulate a response, which will always be a draft and never a final version. But it may help clarify some things that I know my non-trans* friends struggle with.
I’ve taken a while to respond because this is not an easy question. I’ve also taken a while because the responses so far perpetuate misperceptions that hurt the trans* community. 
Trans people don’t change their gender, they make changes to affirm the gender they are and always have been. 
There is no such thing as a verifiable biological sex: when last did you have your sex scientifically verified? And if we all did that we would be surprised to find that we’re somewhere on a continuum of male, female and intersex and not firmly situated in one of those categories. 
Therefore, ‘Transsexual’ is an outdated word as this is about gender, not sex. 
A transman is not a biological woman. A transman is a man with a man’s body, regardless of whether he has a vagina or breasts or top surgery. He is a man in a man’s body when he says so. 
We are not born into the wrong body. We are born into bodies that society perceives wrongly to be of the opposite gender. 
Trans people don’t choose the opposite sex. It’s not sex, but gender, it’s not the opposite gender but a different way of expressing gender that’s not a binary way of being but a fluid one (we all embody both masculinities and femininities – and THIS is the biggest misunderstanding about gender), and it is not a choice, just as much as sexual orientation is not a choice.
So, after what it is not, what is transgender? 
It is the realisation that gender is not what we have been told it is. It is the realisation that it is far more complex, nuanced and fluid than we have been told; that it is not about body parts or clothes or accessories or behaviours or roles. That it is about how we experience ourselves and then how we choose to inhabit those body parts and assemble our wardrobes and perform those roles. 
Gender is not a tickbox exercise: it is a lived experience that far exceeds the arbitrary concepts of ‘male’ and ‘female’ and ‘man’ and ‘woman.
Transgender is the realisation and celebration that ‘man’ and ‘woman’ cannot be defined and therefore emulated – that would end in caricature. 
Trans* is the realisation and celebration that gender is whatever we make it, whatever we say it is, whatever our performance of it enlarges it to be.
Trans* is freedom from definition. Trans* is the infinite possibility of human experience.