Easton's 1897 Bible Dictionary:

Adullam
   one of the royal cities of the Canaanites, now 'Aid-el-ma (Josh.
   12:15; 15:35). It stood on the old Roman road in the valley of
   Elah (q.v.), which was the scene of David's memorable victory
   over Goliath (1 Sam. 17:2), and not far from Gath. It was one of
   the towns which Rehoboam fortified against Egypt (2 Chr. 11:7).
   It was called "the glory of Israel" (Micah 1:15).

     The Cave of Adullam has been discovered about 2 miles south of
   the scene of David's triumph, and about 13 miles west from
   Bethlehem. At this place is a hill some 500 feet high pierced
   with numerous caverns, in one of which David gathered together
   "every one that was in distress, and every one that was in debt,
   and every one that was discontented" (1 Sam. 22:2). Some of
   these caverns are large enough to hold 200 or 300 men. According
   to tradition this cave was at Wady Khureitun, between Bethlehem
   and the Dead Sea, but this view cannot be well maintained.


Hitchcock's Bible Names Dictionary (late 1800's):

Adullam, their testimony; their prey; their ornament