Second-string and Unknown

adjective

• Being a replacement or substitute for a regular member of a team.
  • usage: "a second-string pitcher"

noun

• An unknown and unexplored region.
  • usage: "they came like angels out the unknown"
  • synonyms: unknown region, terra incognita
• Anyone who does not belong in the environment in which they are found.
  • synonyms: stranger, alien
• A variable whose values are solutions of an equation.
  • synonyms: unknown quantity

adjective

• Not known.
  • usage: "an unknown amount"; "an unknown island"; "an unknown writer"; "an unknown source"
• Being or having an unknown or unnamed source.
  • usage: "a poem by an unknown author"; "corporations responsible to nameless owners"; "an unnamed donor"
  • synonyms: nameless, unidentified, unnamed
• Not known to exist.
  • usage: "things obscurely felt surged up from unknown depths"
• Not famous or acclaimed.
  • usage: "an obscure family"; "unsung heroes of the war"
  • synonyms: obscure, unsung
• Not known before.
  • usage: "used many strange words"; "saw many strange faces in the crowd"; "don't let anyone unknown into the house"
  • synonyms: strange

(a.) Not known; not apprehended.

(DoD) An unidentified target. An aircraft or ship that has not been determined to be hostile, friendly, or neutral using identification friend or foe and other techniques, but that must be tracked by air defense or naval engagement systems.
(DoD) A code meaning "information not available."
(DoD) An identity applied to an evaluated track that has not been identified. See also assumed friend; friend; hostile; neutral; suspect.