gender / sex
Social category, or grammatical class, vs. biological class.
Sex is the biological class: male or female. Gender is the social category, and it is also the grammatical class of a noun. Gender is also widely used to mean sex, as on forms that ask for age and gender, and this use is standard.
Examples
Use The vet checked the sex of the chick. (the biological class)
Use The form asks for your age and gender. (gender meaning sex)
Use French nouns have grammatical gender. (the grammatical class)