Source : Webster's Revised Unabridged Dictionary (1913)

Den \Den\, n. [AS. denn; perh. akin to G. tenne floor, thrashing
   floor, and to AS. denu valley.]
   1. A small cavern or hollow place in the side of a hill, or
      among rocks; esp., a cave used by a wild beast for shelter
      or concealment; as, a lion's den; a den of robbers.

   2. A squalid place of resort; a wretched dwelling place; a
      haunt; as, a den of vice. ``Those squalid dens, which are
      the reproach of great capitals.'' --Addison.

   3. Any snug or close retreat where one goes to be alone.
      [Colloq.]

   4. [AS. denu.] A narrow glen; a ravine; a dell. [Old Eng. &
      Scotch] --Shak.

Den \Den\, v. i.
   To live in, or as in, a den.

         The sluggish salvages that den below.    --G. Fletcher.

Source : WordNet®

den
     n 1: the habitation of wild animals [syn: {lair}]
     2: a hiding place; usually a remote place used by outlaws [syn:
         {hideout}, {hideaway}]
     3: a unit of 8 to 10 cub scouts
     4: a room that is comfortable and secluded
     [also: {denning}, {denned}]