From BiblePortal Wikipedia

King James Dictionary [1]

DEEP, a.

1. Extending or being far below the surface descending far downward profound opposed to shallow as deep water a deep pit or well. 2. Low in situation being or descending far below the adjacent land as a deep valley. 3. Entering far piercing a great way. A tree in a good soil takes deep root. A spear struck deep into the flesh. 4. Far from the outer part secreted.

A spider deep ambushed in her den.

5. Not superficial or obvious hidden secret.

He discovereth deep things out of darkness.  Job 12

6. Remote from comprehension.

O Lord, thy thoughts are very deep. Ps. Xcii.

7. Sagacious penetrating having the power to enter far into a subject as a man of deep thought a deep divine. 8. Artful contriving concealing artifice insidious designing as a friend, deep, hollow treacherous. 9. Grave in sound low as the deep tones of an organ. 10. Very still solemn profound as deep silence. 11. Thick black not to be penetrated by the sight.

Now deeper darkness brooded on the ground.

12. Still sound not easily broken or disturbed.

The Lord God caused a deep sleep to fall on Adam.  Genesis 2

13. Depressed sunk low, metaphorically as deep poverty. 14. Dark intense strongly colored as a deep brown a deep crimson a deep blue. 15. Unknown unintelligible.

A people of deeper speech than thou canst perceive.  Isaiah 33

16. Heart-felt penetrating affecting as a deep sense of guilt. 17. Intricate not easily understood or unraveled as a deep plot or intrigue.

This word often qualifies a verb, like an adverb.

Drink deep, or taste not the Pierian spring.

DEEP, n.

1. The sea the abyss of waters the ocean.

He maketh the deep to boil like a pot. Job x1i.

2. A lake a great collection of water.

Lanch out into the deep, and let down your nets.  Luke 5

3. That which is profound, not easily fathomed, or incomprehensible.

Thy judgments are a great deep.  Psalms 36

4. The most still or solemn part the midst as, in deep of night.

Webster's Dictionary [2]

(1): ( superl.) 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.

(2): ( n.) That which is deep, especially deep water, as the sea or ocean; an abyss; a great depth.

(3): ( n.) That which is profound, not easily fathomed, or incomprehensible; a moral or spiritual depth or abyss.

(4): ( adv.) To a great depth; with depth; far down; profoundly; deeply.

(5): ( superl.) Low in situation; lying far below the general surface; as, a deep valley.

(6): ( superl.) Muddy; boggy; sandy; - said of roads.

(7): ( superl.) Of low tone; full-toned; not high or sharp; grave; heavy.

(8): ( superl.) Strongly colored; dark; intense; not light or thin; as, deep blue or crimson.

(9): ( superl.) Profound; thorough; complete; unmixed; intense; heavy; heartfelt; as, deep distress; deep melancholy; deep horror.

(10): ( superl.) Of penetrating or far-reaching intellect; not superficial; thoroughly skilled; sagacious; cunning.

(11): ( superl.) Hard to penetrate or comprehend; profound; - opposed to shallow or superficial; intricate; mysterious; not obvious; obscure; as, a deep subject or plot.

(12): ( superl.) 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.

Fausset's Bible Dictionary [3]

 Romans 10:7, "who shall descend into the deep?" A proverb for impossibility: "say not in thine heart, I wish one could bring Christ up from the dead, but it is impossible." Nay, salvation "is nigh thee," only "believe" in the Lord Jesus raised from the dead, "and thou shalt be saved." Greek abyss ( Luke 8:31), literally, the bottomless place. Translated in  Revelation 9:1-2;  Revelation 9:11;  Revelation 11:7;  Revelation 11:17, "bottomless pit." The demons in the Gadarene besought not to be cast into the abyss, i.e. before their time, the day of final judgment.  2 Peter 2:4; they are "delivered into chains of darkness, and reserved unto judgment."

They are free to hurt meanwhile, like a chained beast, only to the length of their chain ( Judges 1:6). The "darkness of this present world," the "air" ( Ephesians 2:2), is their peculiar element; they look forward with agonizing fear to their final torment in the bottomless pit ( Revelation 20:10). Language is used as though the abyss were in the lowest depth of our earth. We do not know whether this is literal, or an accommodation to human conceptions, to express the farthest removal from the heavenly light.

Wilson's Dictionary of Bible Types [4]

 Psalm 36:6 (b) This word describes the mysteries of GOD which cannot be fathomed.

 Psalm 69:2 (b) Here is a description of the terrible sufferings of Christ

 Daniel 2:22 (b) This is a figure to describe the marvelous mysteries of GOD which cannot be discovered or understood except by divine revelation.

 Romans 10:7 (a) Undoubtedly this word refers to "Sheol" of the Old Testament.

Easton's Bible Dictionary [5]

 Romans 10:7 Luke 8:31 Psalm 69:15 Genesis 1:2 Revelation 9:1,2 11:7 20:13

Hastings' Dictionary of the Bible [6]

DEEP . See Abyss.

International Standard Bible Encyclopedia [7]

(תּהום , tehōm  ; ἄβυσσος , ábussos ,  Luke 8:31 the King James Version;   Romans 10:7 the King James Version; βάθος , báthos ,  Luke 5:4; βυθός , buthós ,  2 Corinthians 11:25 ): The Hebrew word ("water in commotion") is used (1) of the primeval watery waste ( Genesis 1:2 ), where some suggest a connection with Babylonian Tiamat in the creation-epic; (2) of the sea ( Isaiah 51:10 and commonly); (3) of the subterranean reservoir of water (  Genesis 7:11;  Genesis 8:2;  Genesis 49:25;  Deuteronomy 33:13;  Ezekiel 31:4 , etc.). In the Revised Version (British and American) the Greek word first noted is rendered, literally, "abyss." See Abyss; also Astronomy , III, 7.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [8]

(the representative in the A. V. of several Heb. words, especially תְּהוֹם , Tehom' ,  Genesis 1:2, etc. an abyss, often rendered "depth;" Ἄβυσσος ,  Luke 8:31;  Romans 10:7, elsewhere "bottomless pit'). The deep, or the great deep, in its literal sense, signifies, chiefly in Scripture

1. Hell, the place of punishment, the bottomless pit ( Luke 8:31;  Revelation 9:1;  Revelation 11:7).

2. The common receptacle of the dead; the grave, the deep or depths of the earth, under which the body is deposited: the state of the soul corresponding thereto, still more unseen, still deeper, still further distant from human inspection, is that remote country, that "bourne from whence no traveler returns" ( Romans 10:7).

3 . The deepest parts of the sea.( Psalms 49:15;  Psalms 107:26).

4. The chaos, which, in the beginning of the world, was unformed and vacant ( Genesis 1:2). (See Abyss).