directional, noun, numeral · one side of a physical object or one aspect of a situation or matter

这枚硬币的一面印着国徽。

adverb · describing two actions performed at the same time, typically in the '一面 yī​miàn...一面 yī​miàn...' construction

他喜欢一面喝咖啡，一面看报纸。

numeral · across the entire surface of one's face

他吓得一面冷汗。