From BiblePortal Wikipedia

Smith's Bible Dictionary [1]

Apol'lyon. Apollyon or, as it is literally in the margin of the Authorized Version of  Revelation 9:11, "A Destroyer", is the rendering of the Hebrew word, Abaddon , "The Angel Of The Bottomless Pit". From the occurrence of the word in  Psalms 88:11, the rabbins have made Abaddon, the nethermost of the two regions into which they divide the lower world; but that in  Revelation 9:11, Abaddon is the angel and not the abyss, is perfectly evident in the Greek.

Fausset's Bible Dictionary [2]

("destroyer".) Satan ( Revelation 9:11. He is the tempter, in order that he may be at last the destroyer. The Greek translation of the Hebrew Abaddon , (destruction). As the twofold names Αbba (Hebrew) Father (Greek) in  Mark 14:36 combine Jew and Gentile in the common salvation, so Satan's two names Abaddon (Hebrew) and Αpollos (Greek) combine them in a common destruction.

Hastings' Dictionary of the Bible [3]

APOLLYON (‘the Destroyer’). The Greek equivalent in   Revelation 9:11 of Abaddon , the angel of the bottomless pit, who was also the king of the locusts (see Abaddon). The word does not appear in its Greek form in later Rabbinic writings, and only here in the NT. As an angel Apollyon seems to have been regarded as equivalent to Asmodæus, king of demons, in Judaistic mythology; but our data are too few to warrant precise statements.

Shailer Mathews.

Morrish Bible Dictionary [4]

The Greek translation of the Hebrew name ABADDON, which signifies 'destroyer.' He is king of the locusts of the bottomless pit, and ruler over the destroying agents that proceed from thence: it is one of the characters of Satan.  Revelation 9:11 .

King James Dictionary [5]

APOL'LYON, n. Gr. destroying.

The destroyer a name used  Revelation 9:11 , for the angel of the bottomless pit, answering to the Hebrew Abaddon.

Holman Bible Dictionary [6]

 Revelation 9:11Abaddon

Webster's Dictionary [7]

(n.) The Destroyer; - a name used (Rev. ix. 11) for the angel of the bottomless pit, answering to the Hebrew Abaddon.

People's Dictionary of the Bible [8]

Apollyon.  Revelation 9:11. See Abaddon.

Baker's Evangelical Dictionary of Biblical Theology [9]

See Abaddon

Easton's Bible Dictionary [10]

 Revelation 9:11

Watson's Biblical & Theological Dictionary [11]

See Abaddon .

American Tract Society Bible Dictionary [12]

See ABADDON, or Apollyon.

Hastings' Dictionary of the New Testament [13]

See Abaddon.

International Standard Bible Encyclopedia [14]

a - pol´i - on ( Ἀπολλύων , Apollúōn  ; אבדּון , 'ăbhaddōn , "destroyer"): Present participle of the verb ἀπολλύω , "to destroy."

I. Definition

A proper name, original with the author of the Apocalypse and used by him once ( Revelation 9:11 ) as a translation of the Hebrew word "Abaddon" (see Abaddon ) to designate an angel or prince of the lower world.

II. Old Testament Background

1. Fundamental Meaning

The term Abaddon ("destruction") appears solely in the Wisdom literature of the Old Testament and in the following narrow range of instances:  Job 26:6;  Job 28:22;  Job 31:12;  Psalm 88:11;  Proverbs 15:11 . In all these passages save one ( Job 31:12 ) the word is combined either with Sheol, "death," or "the grave," in such a way as to indicate a purely eschatological term based upon the advanced idea of moral distinctions in the realm of the dead. In the one exceptional passage ( Esther 8:6 is incorrectly referred to - the word here is different, namely, אבדן , 'ābhedhān ) where the combination does not occur, the emphasis upon the moral element in the "destruction" mentioned is so definite as practically to preclude the possibility of interpreting the term in any general sense (as Charles, HDB , article "Abaddon"; per con., Briggs, ICC , "Psalms" in the place cited.; BDB , sub loc.). The meaning of the word, therefore, is: the place or condition of utter ruin reserved for the wicked in the realm of the dead.

2. Personification

One other feature of Old Testament usage is worthy of consideration as throwing light upon  Revelation 9:11 . Abaddon and the accompanying terms "Death" and Sheol are personified (as in  Job 28:22 ) and represented as living beings who speak and act (compare  Revelation 6:8 ).

III. New Testament Usage

1. The Starting-Point

The starting-point of the Apocalyptist's use of "Apollyon" is to be found in the fundamental meaning of "Abaddon" as moral destruction in the underworld, together with the occasional personification of kindred terms in the Old Testament. The imagery was in general terms familiar while the New Testament writer felt perfectly free to vary the usage to suit his own particular purposes.

2. Apollyon Not Satan but Part of an Ideal Description

(1) Since Apollyon is a personification he is not to be identified with Satan (compare  Revelation 9:1 where Satan seems to be clearly indicated) or with any other being to whom historical existence and definite characteristics are ascribed. He is the central figure in an ideal picture of evil forces represented as originating in the world of lost spirits and allowed to operate destructively in human life. They are pictured as locusts, but on an enlarged scale and with the addition of many features inconsistent with the strict application of the figure (see   Revelation 9:7-10 ). The intention is, by the multiplication of images which the author does not attempt to harmonize, to convey the impression of great power and far-reaching destructiveness. (2) This interpretation finds additional support in the writer's significant departure from the familiar usage. In the Old Testament the place of destruction is personified - in   Revelation 9:11 , personal forces issue from the Abyss, of which the presiding genius is Destruction in person. The seer's picture is equally independent of the tradition represented by the Talmud (Shab f. 55) where Abaddon is personified as jointly with Death president over six destroying angels. These modifications are evidently due to the exigencies of the pictorial form. It is clearly impossible to portray forces proceeding from the place of ruin in the charge of the place itself.

3. Apollyon Necessary to the Picture

The importance of the conception of Apollyon to the completeness of the picture should not be overlooked. It is intended to represent these forces as having a certain principle of internal unity and as possessors of the power of effective leadership.

4. General Significance of the Description

As to the specific significance of the vision of the locusts as a whole it is not easy to reach a conclusion. Professor Swete suggests ( Commentary on Apocalypse in the place cited.) that "the locusts of the abyss may be the memories of the past brought home at times of divine visitation; they hurt by recalling forgotten sins." It seems to us more probable that it represents an actual historical movement, past or to come, demoniacal in origin and character, human in the mode of its operation and the sphere of its influence, used by God for a scourge upon mankind and kept in restraint by His grace and power. See Abaddon .

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [15]

( Ἀπολλύων ), the Greek equivalent ( Revelation 9:11) of the Hebrews title ABADDON (See Abaddon) (q.,v.).

The Nuttall Encyclopedia [16]

The destroying angel, the Greek name for the Hebrew Abaddon.