Hastings' Dictionary of the Bible 
CÅ’LE-SYRIA , ‘Hollow Syria,’ is properly the great hollow running N. and S. between the Lebanon and Anti-Lebanon ranges ( 1Es 4:48; Strabo, xvi. 2). It corresponds to the Biq‘ath ha-LebÃ¢nÃ´n of Joshua 11:17 etc.; possibly also to Biq‘ath Aven of Amos 1:5 . The first element of the name persists in the modern name of the valley S. of Baalbek, el-BuqÃ¢‘ . The Orontes drains the valley northward, and the LitÃ¢ni southward, both rivers rising near Baalbek. The soil is rich, producing splendid crops of wheat, etc., while some of the finest vineyards in Syria clothe the adjoining slopes.
‘CÅ“le-Syria’ came to have a wider significance, covering indeed, with PhÅ“nicia, all the Seleucid territory S. of the River Eleutherus ( 2M Malachi 3:5 etc.; Strabo, xvi. 753). In 1E Esther 2:17 etc., CÅ“le-Syria and PhÅ“nicia denote the whole Persian province, stretching from the Euphrates to the borders of Egypt. Josephus reckons the country E. of Jordan to CÅ“le-Syria ( Ant . I. xi. 5, XIII. xiii. 2 f.,etc.), including in it Scythopolis, the only member of the Decapolis west of the river.
Smith's Bible Dictionary 
Coele-Syr'ia. (Hollow Syria). The remarkable valley or hollow which intervenes between Libanus and Anti-Libanus, stretching a distance of nearly a hundred miles. The only mention of the region as a separate tract of country, which the Jewish Scriptures contain, is probably that in Amos 1:5, where "the inhabitants of the plain of Aven" are threatened in conjunction with those of Damascus. The word is given in the Authorized Version, as Celo-Syria .
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
( Ἡ Κοίλη Συρία ; Vulg. Celesyria ) , "the Hollow Syria," was (strictly speaking) the name given by the Greeks, in the times of the Seleucidae, to the remarkable valley or hollow ( Κοιλία ) which intervenes between Libanus and Anti-Libanus, stretching from lat. 33 ° 20' to 34 ° 40', a distance of nearly a hundred miles. As applied to this region the word is strikingly descriptive (see Dionysius, Perieg. 899-900). Thus a modern traveler observes: "We finally looked down on the vast green and red valley — green from its yet unripe corn, red from its vineyards not yet verdant — which divides the range of Lebanon and Anti-Lebanon; the former reaching its highest point in the snowy crest to the north, behind which lie the Cedars; the latter in the still more snowy crest of Hermonthe culmination of the range being thus in the one at the northern, in the other at the southern extremity of the valley which they bound. The view of this great valley is chiefly remarkable as being exactly to the eye what it is on maps — the ‘ hollow' between the two mountain ranges of Syria. A screen through which the Leontes (Litany) breaks out closes the south end of the plain. There is a similar screen at the north end, but too remote to be visible" (Stanley's Palestine, p. 399). The plain gradually rises towards its center, near which, but a little on the southern declivity, stand the ruins of Baalbek or Heliopolis. In the immediate neighborhood of Baalbek rise the two streams of the Orontes (Nahr-el-Asy) and the Litany, which, flowing in opposite directions to the north-west and the south-east, give freshness and fertility to the tract enclosed between the mountain ranges. Amyce, the name of the plain through which the Orontes flowed ( Τὸ Ἀμύκης Πεδίον , Polyb. v. 59), is derived by Bochart from the Syriac עמיקא , Amica, which means Deep , and is nearly synonymous with the Greek Caele ( Geogr. Sac . I, 1, 1).
The term Coele-Syria was also used in a much wider sense. In the first place it was extended so as to include the inhabited tract to the east of the Anti-Libanus range, between it and the desert, in which stood the great city of Damascus; and then it was further carried on upon that side of Jordan, through Trachonitis and Peraea, to Idumaea and the borders of Egypt (Strab. 16, § 21; Polyb. v. 80, § 3; Josephus, Ant. 1:11, 5). Ptolemy (v. 15) and Josephus (Ant. 13:13, 2) even place Scythopolis in Coele-Syria, though it was upon the west side of Jordan; but they seem to limit its extent southwards to about lat. 31 ° 30', or the country of the Ammonites (Ptol. v. 15; Josephus, Ant. 1:11, 5). Ptolemy distinctly includes in it the Damascus country. In the time of David, Caele-Syria was probably included in "Syria of Damascus," which was conquered by that monarch ( 2 Samuel 8:6), but recovered from Solomon by Rezon, the son of Eliadah ( 1 Kings 11:24). The possession of it was an object of many struggles between the Seleucidae and the kings of Egypt (Polyb. 1:3; 2:71; 3:1; v. 40; 16:39; 27:17).
There can be little doubt that a part at least of Coele-Syria was included in that "Valley of Lebanon" ( בַּקְעִת הִלְּבָנוֹן ) mentioned by Joshua ( Joshua 11:17; Joshua 12:7), the extent of which has been too much restricted by recent geographers. The name "Valley of Lebanon" could scarcely be applied with propriety Exclusively to that section of the great valley which lies at the base of Hermon, at a considerable distance from the range of Lebanon. Doubtless Baal-Gad was situated "under Mount Hermon;" but we have reason to believe that the "Valley of Lebanon" includes the whole of that valley which separates the ridge of Hermon from that of Lebanon. It seems that at a subsequent period this valley was called by Amos, apparently in contempt, "the valley of idols" ( בַּקְעִת אָוֶן , Amos 1:5). SEE AVEN. The name was most appropriate. The whole sides of the valley are thickly studded with old heathen temples. Mr. Porter visited no less than fourteen of them, and he heard of several others. Some of them were of- great size and splendor, such as those of Baalbek, Mejdel, Niha, and Hibbariyeh. This appears, in fact, to have been the chosen house of idolatry (Porter's Damascus, 1:12; 2:320; Hand-Book Of S. And P. p. 568, 570; Robinson, Later Bib. Res . p. 438, 492, 520). The modern name of the valley confirms the above view. It is called El-Bukaa , which is strictly the same as the Hebrews Bikah ( בַּקְעָה ).
In the apocryphal books there is frequent mention of Coele-Syria in a somewhat vague sense, nearly as an equivalent for Syria ( 1 Esdras 2:17; 1 Esdras 2:24; 1 Esdras 2:27; 1 Esdras 4:48; 1 Esdras 6:29; 1 Esdras 7:1; 1 Esdras 8:67; 1 Maccabees 10:69; 2 Maccabees 3, 5, 8; 2 Maccabees 4:4; 2 Maccabees 8:8; 2 Maccabees 10:11). In all these cases the word is given in the A. V. as "Celo- Syria," i.e. Coele-Syria. In Ezra 6:3, it is called simply "Syria." Under the emperor Diocletian, Phoenice and Coele-Syria formed one province, called Phoenicia Libanica. Under the present Turkish government the western part of Coele-Syria is in the pashalic of Saide, and the eastern in the pashalic of Damascus. (See Syria).
International Standard Bible Encyclopedia 
sē - lē̇ - sir´i - a (the King James Version Celosyria; Κοίλη Συρία , Koı́lē Surı́a , "hollow Syria"): So the Greeks after the time of Alexander the Great named the valley lying between the two mountain ranges, Lebanon and Anti-Lebanon. It is referred to in the Old Testament as נ , Biḳ‛ath ha - Lebhānōn , "the valley of Lebanon" ( Joshua 11:17 ), a name the echo of which is still heard in el - Buḳā‛ , the designation applied today to the southern part of the valley. This hollow, which extends about 100 miles in length, is the continuation northward of the Jordan valley. The main physical features are described under Lebanon (which see). The name, however, did not always indicate the same tract of territory. In Strabo (xvi.2) and Ptolemy (v.15), it covers the fertile land between Jebel esh - Sharḳy and the desert presided over by Damascus. In 1 Esdras 2:17; 2 Macc 3:8, etc., it indicates the country South and East of Mt. Lebanon, and along with Phoenicia it contributed the whole of the Seleucid dominions which lay South of the river Eleutherus. Josephus includes in Coele-Syria the country east of the Jordan, along with Scythopolis (Beisan) which lay on the West, separated by the river from the other members of the Decapolis ( Ant. , Xiii , xiii, 2, etc.). In Xiv , iv, 5, he says that "Pompey committed Coele-Syria as far as the river Euphrates and Egypt to Scaurus." The term is therefore one of some elasticity.
The Nuttall Encyclopedia 
Howe of Syria), or
valley between the Lebanons, about 100 m. long by 10 m. broad.