This is the 5177th most frequent Russian word.
неизвестным
"Unknown."
Adjective describing 'человека' (person) as being unknown.
Я встретил неизвестного человека на улице.
I met an unknown person on the street.
Adjective describing 'причина' (reason) as being unknown.
Причина его поступка осталась неизвестной для нас.
The reason for his action remained unknown to us.
Used as a noun in the neuter plural form to refer to individuals who are unknown.