That's the thing ACat, though, it has no completely agreed definition.
I mean, obviously, we're in agreement that the way you choose to present yourself to the world is not your sex, which is what feminists have been saying for a long time.
An indefinable inner sense of identity is clearly important to some people, and those who experience a sense of detachment and distress about their bodies obviously feel that certain things help them feel better.
But some people are saying a man wearing things gendered 'feminine' in our culture is evidence of Transness, and even though the distinction is made between gender identity and gender presentation, the presentation is obvious significant to many people, and none of it has any bearing on understanding a nebulous and somehow separate gender identity that we are told has nothing to do with gender expression...