The Collaborative International Dictionary of English v.0.48:

Den \Den\ (d[e^]n), 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.
      [1913 Webster]

   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.
      [1913 Webster]

   3. Any snug or close retreat where one goes to be alone.
      [Colloq.]
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   4. [AS. denu.] A narrow glen; a ravine; a dell. [Old Eng. &
      Scotch] --Shak.
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The Collaborative International Dictionary of English v.0.48:

Den \Den\, v. i.
   To live in, or as in, a den.
   [1913 Webster]

         The sluggish salvages that den below.    --G. Fletcher.
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WordNet (r) 3.0 (2006):

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