The Cave of Addulam was originally an underground cavern referred to in the Old Testament in which David, already anointed to succeed Saul as king, sought refuge from the latter (e.g. 1 Sam 22 ff), however it is used more generally to refer to groups of political outsiders plotting their comeback or the overthrow of the status quo, especially after recent defeat.
Adullam is a town referred to in the Hebrew Bible.
The first book of Samuel refers to the Cave of Adullam, and reports that David, when he had been expelled from the court of King Saul, there gathered together "every one that was in distress, and every one that was in debt, and every one that was discontented" (1 Sam.
Adullam today describes a region of Israel near the Valley of Elah (to the south of Bet Shemesh), east of Gush Etzion.