The Collaborative International Dictionary of English v.0.48:

Unseen \Un*seen"\, a.
   1. Not seen or discovered.
      [1913 Webster]

   2. Unskilled; inexperienced. [Obs.] --Clarendon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unseen
    adj 1: not observed [syn: unobserved, unseen]
    n 1: a belief that there is a realm controlled by a divine
         spirit [syn: spiritual world, spiritual domain,
         unseen]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "unseen":
   behind the curtain, behind the scenes, camouflaged, concealed,
   disguised, hidden, imperceptible, indiscernible, insensible,
   invisible, latent, out of sight, secret, sightless, submerged,
   unapparent, unbeheld, unbeholdable, unconsidered, undescried,
   undetected, undiscernible, undisclosed, undiscovered, undivulged,
   unexplained, unexplored, unexposed, unheeded, unmarked, unminded,
   unmissed, unnoted, unnoticed, unobserved, unperceivable,
   unperceived, unrealized, unregarded, unremarked, unrevealed,
   unseeable, unsolved, unspied, unthought-of, untraced, untracked,
   unviewed, unwitnessed, viewless