The sex you are born with, based on your sexual organs alone. A child born with a penis has a male birth sex, a child born with a vagina has a female birth sex, and a child born with some form of combination or lack of the two has an intersex birth sex.
Generally used for those who in some way work against their birth sex, especially in the transgender community.
"His birth sex is female, but because he is a transsexual and has transitioned, he is now in every way male."
"Just because her birth sex says she's male doesn't mean she actually is a guy."