not known or familiar

The exact number of people carrying the virus is unknown. 病毒携带者的准确数目还不清楚。

As recently as six months ago her name was almost unknown in this country. 就在6个月前，她的名字在这个国家还几乎无人知晓。

Unknown to me, she'd organized a party for my birthday. 她为我组织了生日派对，我却还蒙在鼓里。

what is not familiar or known

Racism is in some ways just a fear of the unknown. 在某些意义上，种族主义只是一种对未知事物的恐惧罢了。

a person, especially a performer or sports player, who is not famous

For her latest film she deliberately chose a cast of unknowns. 在新拍影片中，她特意挑了一批不知名的演员。

something that cannot be guessed at or calculated because so little is known about it

It's the big unknowns that make insurance companies uneasy. 令保险公司担心的是无法预测的重大事件。