The sex (male, female, intersex) assigned to a child at birth, most often based on the child's external anatomy. Also referred to as birth sex, natal sex, biological sex, or sex.
Other examples: AFAB (assigned female at birth), AMAB (assigned male at birth).
Note: It is not valid to use “biology” or “science” to claim assigned sex at birth and/or gender are a fixed, concrete concept and/or binary. Science favors the fact that sex and gender are more complicated and involve much more than one’s sexual organs.
Adapted from The Fenway Institute and informed by Stop Using Phony Science to Justify Transphobia - Scientific American Blog Network