Fausset's Bible Dictionary 
Hebrew for KJV "the vale," "the plain," "the low country"; rather, as 1 Maccabees 12:38 proves, the low hills between the central mountains and the seacoast plain, compare Seville; for Adida on the Shephelah answers to Haditheh, which is not in the plain but the low hills. The valleys amidst the Shephelah are seldom more than 300 ft. deep, and the slopes much more gradual. Eusebius says that the country about Eleutheropolis was still called Shephelah . It is the district of rolling hills, not spurs or shoulders from the main range, but between this and the plain below.
The article is always prefixed, the Shephelah ( Ha-Shephelah ), a "marked physical feature of the land"; like our phrase "the downs," "the wolds" ( Zechariah 7:7; Joshua 15:33; Deuteronomy 1:7). The divisions are mountain, "hill" Shephelah , and plain (Talmud, tract Shevith). Rabbi Jochanan says that from Bethhoron to Emmaus is "mountain" ( Har ); from Emmaus to Lydda hill; and from Lydda to the sea plain. In Joshua 15:33-47 the Shephelah contains 42 cities with their dependent hamlets, many of them in the mountains. The Shephelah is most fruitful, receiving, as it does, the soil washed down from the mountains behind by the winter rains; and here were extensive tracts of grain land, the references to which and to the flails and other agricultural instruments are frequently met with.
Bridgeway Bible Dictionary 
‘Shephelah’ is a Hebrew word used to describe the low hill country between the coastal plain and the central mountains of Palestine. It was a fertile region, suitable for growing fruit trees and raising sheep and cattle ( Joshua 10:40; Judges 1:9; 2 Chronicles 9:27; 2 Chronicles 26:10; Jeremiah 17:26; Jeremiah 32:44). For details see Palestine sub-heading ‘Shephelah’.
Holman Bible Dictionary 
Hastings' Dictionary of the Bible 
Shephelah . See Plain (5).
International Standard Bible Encyclopedia 
shef - ē´la ( השּפלה , ha - shephēlāh ; σεφηλά , sephēlá , σαφηλά , saphēlá ):
1. Name and References:
The word denotes "lowland," and is variously rendered in the King James Version. It is "vale" in Deuteronomy 1:7; Joshua 10:40; 1 Kings 10:27; 2 Chronicles 1:15; Jeremiah 33:13; "valley" in Joshua 9:1; Joshua 11:2 , Joshua 11:16; Joshua 12:8; Joshua 15:33; Judges 1:9; Jeremiah 32:44; "low plain" in 1 Chronicles 27:28; 2 Chronicles 9:27; "plain" in Jeremiah 17:26; Obadiah 1:19; Zechariah 7:7; and "low country" in 2 Chronicles 28:18 . the Revised Version (British and American) renders uniformly "lowland." As the word always occurs with the definite article, indicating a distinct district, it might have been well to retain it without translation. The boundaries of the district are clearly marked and include much broken country; the hills being low compared with the mountains to the East, but much higher than the plain that runs to the shore. If a translation was to be made, perhaps "lowlands" would have been the best, as applied to the "Lowlands" of Scotland, "which likewise are not entirely plain, but have their groups and ranges of hills" ( HGHL , 203). In the wide sense the Shephelah included the territory originally given to the tribe of Dan, and also a considerable part of Western and Southwestern Judea. At an early day the tribes of Dan and Simeon were practically absorbed by Judah, and hence, we find in Josh 15 many cities in the Shephelah which belonged to that tribe ( LB , I, 211).
2. Districts and Features:
(1) The sites of many ancient cities named in the Shephelah have been identified. They all lie within the strip of hill country that runs along the western base of the mountains of Judah, terminating in the North at the Valley of Aijalon. Once indeed the name appears to apply to the low hills North of this ( Joshua 11:16 , 'the mount of Israel and its Shephelah'). Every other reference applies only to the South.
Principal G. A. Smith has pointed out the difference between the district to the N. and that to the S. of Aijalon ( Hghl , 203 ff). "North of Ajalon the low hills which run out on Sharon are connected with the high mountains behind them. You ascend to the latter from Sharon either by long sloping ridges, such as that which today carries the telegraph wire and the high road from Jaffa to Nāblus ; or else you climb up terraces, such as the succession of ranges closely built upon one another by which the country rises from Lydda to Bethel. That is, the low hills west of Samaria are (to use the Hebrew phrase) 'ăshēdhōth , or slopes of the central range, and not a separate group. But South of Ajalon the low hills do not so hang upon the Central Range, but are separated from the mountains of Judah by a series of valleys, both wide and narrow, which run all the way from Ajalon to near Beersheba; and it is only when the low hills are thus flung off the Central Range into an independent group, separating Judea from Philistia, that the name Shephelah seems to be applied to them."
(2) On the East of the Shephelah, then, taking the name in this more limited sense, rises the steep wall of the mountain, into which access is gained only by narrow and difficult defiles. The hills of the Shephelah are from 500 to 800 ft. high, with nothing over 1,500. The formation is soft limestone. In the valleys and upland plains there is much excellent land which supports a fairly good population still. Wheat, barley and olives are the chief products. But ancient wine presses cut in the rocks testify to the culture of the vine in old times. The district is almost entirely dependent on the rain for its water-supply. This is collected in great cisterns, partly natural. The rocks are in many places honeycombed with caves.
The western boundary is not so definite as that on the East. Some have held that it included the Philistine plain. This contention draws support from the mention of the Philistine cities immediately after those of Judah, which are said to be in the Shephelah ( Joshua 15:45 ff; these verses can hardly be ruled out as of a later date). On the other hand the Philistines are said to have invaded the cities of the Shephelah ( 2 Chronicles 28:18 ), which implies that it was outside their country. In later times the Talmud (Jerusalem, Shebhı̄‛ı̄th 9 2) distinguishes the Mountain, the Shephelah, and the Plain. See, however, discussion in Buhl ( GAP , 104, n.; and G. A. Smith, The Expositor , 1896, 404 ff).
3. The Five Valleys:
The Shephelah is crossed by five wide valleys which furnish easy access from the plain. These are of importance chiefly because from each of them a way, crossing the "foss," enters one of the defiles by which alone armies could approach the uplands of Judea. The hills of Judea are much steeper on the east than on the west, where they fall toward Philistia in long-rolling hills, forming the Shephelah.
(1) Vale of Aijalon:
The most noteworthy of these is the Vale of Aijalon. It winds its way first in a northeasterly direction, past the Beth-horons, then, turning to the Southeast, it reaches the plateau at el - Jib , the ancient Gibeon, fully 5 miles Northwest of Jerusalem. This is the easiest of all the avenues leading from the plain to the heights, and it is the one along which the tides of battle most frequently rolled from the days of Joshua ( Joshua 10:12 ) to those of the Maccabees (1 Macc 3:16 ff, etc.). It occupies also a prominent place in the records of the Crusades.
(2) Wady es-Surar:
Wâdy eṣ - Surār , the Valley of Sorek, crosses the Shephelah South of Gezer, and pursues a tortuous course past Beth-shemesh and Kiriath-jearim to the plateau Southwest of Jerusalem. This is the line followed by the Jaffa-Jerus Railway.
(3) Wady es-Sunt:
Wâdy eṣ - Sunṭ runs eastward from the North of Tell eṣ - Sāfieh (Gath) up the Vale of Elah to its confluence with Wâdy eṣ - Sur which comes in from the South near Khirbet Shuweikeh (Socoh); and from that point, as Wâdy el - Jindy , pursues its way South of Timnah to the uplands West of Bethlehem.
(4) Wady el-'Afranj:
Wâdy el - ‛Afranj crosses the plain from Ashdod (Esdud), passes Beit Jibrı̄n (Eleutheropolis), and winds up through the mountains toward Hebron.
(5) Wady el-Hesy:
Wâdy el - Ḥesy , from the sea about 7 miles North of Gaza, runs eastward with many windings, passes to the North of Lachish, and finds its way to the plateau some 6 miles Southwest of Hebron.
From the Shephelah thus opened the gateways by which Judea and Jerusalem might be assailed: and the course of these avenues determined the course of much of the history. It is evident that the shephelah lay open to attack from both sides, and for centuries it was the debatable land between Israel and the Philistines. The ark for a time sojourned in this region ( 1 Samuel 5:6 f). In this district is laid the scene of Samson's exploits (Jdg 14 through 16). The scene of David's memorable victory over the giant was in the Wâdy eṣ - Sunt , between Socoh and Azekah ( 1 Samuel 17:1 ). David found refuge here in the cave of Adullam ( 1 Samuel 22:1 ). For picturesque and vivid accounts of the Shephelah and of the part it played in history see Smith, HGHL , 201 ff; A. Henderson, Palestine, Its Historical Geography , 1894.