Definitions of unknown:

• noun:   a variable whose values are solutions of an equation
• noun:   an unknown and unexplored region

  Example: "They came like angels out the unknown"

• noun:   anyone who does not belong in the environment in which they are found
• adjective:   not known

  Example: "An unknown amount"

• adjective:   not known to exist
• adjective:   not known before

  Example: "Don't let anyone unknown into the house"

• adjective:   not famous or acclaimed
• adjective:   being or having an unknown or unnamed source

  Example: "A poem by an unknown author"