1. 面具;面罩;假面
    A mask is a piece of cloth or other material, which you wear over your face so that people cannot see who you are, or so that you look like someone or something else.

    e.g. The gunman, whose mask had slipped, fled.
           面罩掉落的持枪歹徒逃之夭夭了。
    e.g. ...actors wearing masks.
           戴面具的演员们

2. （防细菌或有害物质的）防护面罩,面具，口罩
    A mask is a piece of cloth or other material that you wear over all or part of your face to protect you from germs or harmful substances.

    e.g. You must wear goggles and a mask that will protect you against the fumes...
           为了免受烟雾危害，你必须戴上护目镜和面罩。
    e.g. She wore a surgical mask and rubber gloves while she worked with the samples.
           她处理样品的时候戴着手术口罩和橡胶手套。

3. (掩盖真实感情或性格的)伪装，掩饰
    If you describe someone's behaviour as a mask, you mean that they do not show their real feelings or character.

    e.g. His mask of detachment cracked, and she saw for an instant an angry and violent man.
           他冷静超然的伪装破碎了，一瞬间她的面前出现了一个愤怒而又狂暴的男人。

4. （护肤用的）面膜
    A mask is a thick cream or paste made of various substances, which you spread over your face and leave for some time in order to improve your skin.

mask的翻译

    e.g. This mask leaves your complexion feeling soft and supple.
           这种面膜能使你的皮肤变得柔软细嫩。

5. 掩饰(情感);伪装
    If you mask your feelings, you deliberately do not show them in your behaviour, so that people cannot know what you really feel.

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    e.g. Dena lit a cigarette, trying to mask her agitation.
           德娜点燃了一根烟，试图掩饰不安。

6. 遮住;掩蔽;使…不易察觉
    If one thing masks another, it prevents people from noticing or recognizing the other thing.

    e.g. A thick grey cloud masked the sun...
           一片厚厚的乌云遮住了太阳。
    e.g. Too much salt masks the true flavour of the food...
           太多的盐会盖住食物本来的味道。

7. see also: death mask;gas mask;oxygen mask