Source : Webster's Revised Unabridged Dictionary (1913)

Deep \Deep\ (d[=e]p), a. [Compar. {Deeper}; superl. {Deepest}.]
   [OE. dep, deop, AS. de['o]p; akin to D. diep, G. tief, Icel.
   dj[=u]pr, Sw. diup, Dan. dyb, Goth. diups; fr. the root of E.
   dip, dive. See {Dip}, {Dive}.]
   1. Extending far below the surface; of great perpendicular
      dimension (measured from the surface downward, and
      distinguished from high, which is measured upward); far to
      the bottom; having a certain depth; as, a deep sea.

            The water where the brook is deep.    --Shak.

   2. Extending far back from the front or outer part; of great
      horizontal dimension (measured backward from the front or
      nearer part, mouth, etc.); as, a deep cave or recess or
      wound; a gallery ten seats deep; a company of soldiers six
      files deep.

            Shadowing squadrons deep.             --Milton.

            Safely in harbor Is the king's ship in the deep
            nook.                                 --Shak.

   3. Low in situation; lying far below the general surface; as,
      a deep valley.

   4. Hard to penetrate or comprehend; profound; -- opposed to
      shallow or superficial; intricate; mysterious; not
      obvious; obscure; as, a deep subject or plot.

            Speculations high or deep.            --Milton.

            A question deep almost as the mystery of life. --De
                                                  Quincey.

            O Lord, . . . thy thoughts are very deep. --Ps.
                                                  xcii. 5.

   5. Of penetrating or far-reaching intellect; not superficial;
      thoroughly skilled; sagacious; cunning.

            Deep clerks she dumbs.                --Shak.

   6. Profound; thorough; complete; unmixed; intense; heavy;
      heartfelt; as, deep distress; deep melancholy; deep
      horror. ``Deep despair.'' --Milton. ``Deep silence.''
      --Milton. ``Deep sleep.'' --Gen. ii. 21. ``Deeper
      darkness.'' -->Hoole. ``Their deep poverty.'' --2 Cor.
      viii. 2.

            An attitude of deep respect.          --Motley.

   7. Strongly colored; dark; intense; not light or thin; as,
      deep blue or crimson.

   8. Of low tone; full-toned; not high or sharp; grave; heavy.
      ``The deep thunder.'' --Byron.

            The bass of heaven's deep organ.      --Milton.

   9. Muddy; boggy; sandy; -- said of roads. --Chaucer.

            The ways in that vale were very deep. --Clarendon.

   {A deep line of operations} (Military), a long line.

   {Deep mourning} (Costume), mourning complete and strongly
      marked, the garments being not only all black, but also
      composed of lusterless materials and of such fashion as is
      identified with mourning garments.

Deep \Deep\, n.
   1. That which is deep, especially deep water, as the sea or
      ocean; an abyss; a great depth.

            Courage from the deeps of knowledge springs.
                                                  --Cowley.

            The hollow deep of hell resounded.    --Milton.

            Blue Neptune storms, the bellowing deeps resound.
                                                  --Pope.

   2. That which is profound, not easily fathomed, or
      incomprehensible; a moral or spiritual depth or abyss.

            Thy judgments are a great deep.       --Ps. xxxvi.
                                                  6.

   {Deep of night}, the most quiet or profound part of night;
      dead of night.

            The deep of night is crept upon our talk. --Shak.

Deep \Deep\, adv.
   To a great depth; with depth; far down; profoundly; deeply.

         Deep-versed in books, and shallow in himself. --Milton.

         Drink deep, or taste not the Pierian spring. --Pope.

   Note: Deep, in its usual adverbial senses, is often prefixed
         to an adjective; as, deep-chested, deep-cut,
         deep-seated, deep-toned, deep-voiced, ``deep-uddered
         kine.''

Source : WordNet®

deep
     adj 1: relatively deep or strong; affecting one deeply; "a deep
            breath"; "a deep sigh"; "deep concentration"; "deep
            emotion"; "a deep trance"; "in a deep sleep" [ant: {shallow}]
     2: marked by depth of thinking; "deep thoughts"; "a deep
        allegory"
     3: having great spatial extension or penetration downward or
        inward from an outer surface or backward or laterally or
        outward from a center; sometimes used in combination; "a
        deep well"; "a deep dive"; "deep water"; "a deep
        casserole"; "a deep gash"; "deep massage"; "deep pressure
        receptors in muscles"; "deep shelves"; "a deep closet";
        "surrounded by a deep yard"; "hit the ball to deep center
        field"; "in deep space"; "waist-deep" [ant: {shallow}]
     4: very distant in time or space; "deep in the past"; "deep in
        enemy territory"; "deep in the woods"; "a deep space
        probe"
     5: extreme; "in deep trouble"; "deep happiness"
     6: having or denoting a low vocal or instrumental range; "a
        deep voice"; "a bass voice is lower than a baritone
        voice"; "a bass clarinet" [syn: {bass}]
     7: strong; intense; "deep purple"; "a rich red" [syn: {rich}]
     8: relatively thick from top to bottom; "deep carpets"; "deep
        snow"
     9: extending relatively far inward; "a deep border"
     10: (of darkness) very intense; "thick night"; "thick darkness";
         "a face in deep shadow"; "deep night" [syn: {thick}]
     11: large in quantity or size; "deep cuts in the budget"
     12: with head or back bent low; "a deep bow"
     13: of an obscure nature; "the new insurance policy is written
         without cryptic or mysterious terms"; "a deep dark
         secret"; "the inscrutible workings of Providence"; "in
         its mysterious past it encompasses all the dim origins of
         life"- Rachel Carson; "rituals totally mystifying to
         visitors from other lands" [syn: {cryptic}, {cryptical},
         {inscrutable}, {mysterious}, {mystifying}]
     14: difficult to penetrate; incomprehensible to one of ordinary
         understanding or knowledge; "the professor's lectures
         were so abstruse that students tended to avoid them"; "a
         deep metaphysical theory"; "some recondite problem in
         historiography" [syn: {abstruse}, {recondite}]
     15: exhibiting great cunning usually with secrecy; "deep
         political machinations"; "a deep plot"

deep
     adv 1: to a great depth; "dived deeply"; "dug deep" [syn: {deeply}]
     2: to an advanced time; "deep into the night"; "talked late
        into the evening" [syn: {late}]
     3: to far into space; "penetrated deep into enemy territory";
        "went deep into the woods";

deep
     n 1: the central and most intense or profound part; "in the deep
          of night"; "in the deep of winter"
     2: a long steep-sided depression in the ocean floor [syn: {trench},
         {oceanic abyss}]
     3: literary term for an ocean; "denizens of the deep"