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.]
      [1913 Webster]

   4. [AS. denu.] A narrow glen; a ravine; a dell. [Old Eng. &
      Scotch] --Shak.
      [1913 Webster]

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.
   [1913 Webster]

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

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "den":
   Babylon, Gomorrah, Sodom, adytum, ashram, asylum, bolt-hole,
   booking office, bordello, box office, branch, branch office,
   brothel, burrow, cabinet, cache, cathouse, cave, cell, cesspit,
   cesspool, chambers, chancellery, chancery, clip joint, cloister,
   closet, concealment, consulate, corner, corporate headquarters,
   couch, cover, covert, coverture, cranny, cubby, cubbyhole,
   dark corner, den of iniquity, den of thieves, dive, dugout, dump,
   earth, embassy, executive office, fleshpots, form, foxhole,
   funk hole, gyp joint, headquarters, hellhole, hermitage, hideaway,
   hideout, hidey hole, hiding, hiding place, hole, holy of holies,
   home office, ivory tower, joint, lair, legation, lodge,
   main office, mew, niche, nook, office, pandemonium, privacy,
   recess, refuge, retreat, run, sanctuary, sanctum,
   sanctum sanctorum, secret place, shop, sink, sink of corruption,
   sporting house, stash, stews, study, sty, ticket office, tunnel,
   undercovert, whorehouse



V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

DEN
       Directory Enabled Networking (MS)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

DEN
       Document Enabled Networking (Novell, Xerox)


Easton's 1897 Bible Dictionary:

Den
   a lair of wild beasts (Ps. 10:9; 104:22; Job 37:8); the hole of
   a venomous reptile (Isa. 11:8); a recess for secrecy "in dens
   and caves of the earth" (Heb. 11:38); a resort of thieves (Matt.
   21:13; Mark 11:17). Daniel was cast into "the den of lions"
   (Dan. 6:16, 17). Some recent discoveries among the ruins of
   Babylon have brought to light the fact that the practice of
   punishing offenders against the law by throwing them into a den
   of lions was common.