Gender identity. That indefinable thing that is beyond definition and only exists as a thought process in people's heads which is different for everyone who experiences it, which is why it can't be defined.
It's nothing to do with sex, and it definitely isn't to do with gender stereotypes, and yet it somehow aligns, or doesn't align, with people's sex.
Despite being indefinable and different for everyone who experiences it, somehow we know that it's really the same thing being experienced by all the people who experience it.
Despite being nothing to do with sex, it's people's sex which is the focus of gender identity, and it's genitals and secondary sex characteristics which must be cosmetically altered to align gender identity with sex.
Gender identity. It's so, so important.