A person successfully and convincingly presenting themselves as a cisgender member of a gender they were not assigned at birth is called "passing". Passing is a complicated and controversial topic, partly because of the issues that it raises for both those who can and cannot pass. It's also a philosophical issue, a safety issue, and a social issue (and a variety of other categories).
Gender ≠ Sex
Put simply, a person's sex is between their legs, but their gender is between their ears.
Who am I?
I've recently come to realize that I'm genderfluid. No, I'm not some puddle, and you can't use a mop to clean me up (well I suppose you could, but I wouldn't let you). I'm genderfluid: I experience my sense of my own gender as an frequently-changing place somewhere on the gender spectrum; and sometimes multiple places on the spectrum simultaneously; and sometimes I'm not even on the spectrum. Confused? Me, too!