1. 不知道的；不了解的；未知的
    If something is unknown to you, you have no knowledge of it.

unknown

    e.g. An unknown number of demonstrators were arrested...
           有数目不详的示威者遭到了逮捕。
    e.g. How did you expect us to proceed on such a perilous expedition, through unknown terrain...
           你怎么能指望我们如此冒险地穿越未知的地域，继续这一探险？

2. 不认识的；不熟悉的；陌生的
    An unknown person is someone whose name you do not know or whose character you do not know anything about.

    e.g. Unknown thieves had forced their way into the apartment...
           不知名的窃贼破门闯入了公寓。
    e.g. I could not understand how someone with so many awards could be unknown to me.
           我想不通自己竟然不认识获了这么多奖的人。

3. 不出名的；无人知晓的；未获公众认可的
    An unknown person is not famous or publicly recognized.

    e.g. He was an unknown writer.
           他是个名不见经传的作家。
    e.g. ...a popular environment where both established and unknown artists can meet, talk and drink.
           成名和未成名的艺术家都可以聚会、聊天及喝酒的一处热门场所

4. 从未发生的；从未存在的
    If you say that a particular problem or situation is unknown, you mean that it never occurs.

    e.g. A hundred years ago coronary heart disease was virtually unknown in Europe and America.
           在100年前冠心病几乎不存在于欧洲和美洲。

5. （总称）未知事物，不为人知的地方
    The unknown refers generally to things or places that people do not know about or understand.

    e.g. Ignorance of people brings fear, fear of the unknown.
           人们的无知会带来恐惧，对未知事物的恐惧。