性别 (xìngbié) is the standard, neutral noun for 'gender' or 'sex' of a person, used on forms and in general discussion. It is distinct from 性 (xìng), which as a suffix means '-ness' or '-ity' (e.g., 可能性 'possibility'), and from 女性/男性 (nǚxìng/nánxìng), which refer specifically to women and men as groups. 雄性/雌性 (xióngxìng/cíxìng) are used only for non-human species.