Concise Bible Dictionary:

A district embracing ten cities (as its name implies). After the conquest of Palestine by the Romans these cities were rebuilt and partly colonized, having peculiar privileges. Historians are not quite agreed as to which were the ten cities, but they are now generally held to have been Hippos, Gadara, Pella, Philadelphia, Gerasa, Dion, Canatha, Damascus, Raphana, and Scythopolis. All were on the east of the Jordan except Scythopolis: but the name Decapolis seems to have been used for a district on the west of the Jordan as well as on the east (Matt. 4:2525And there followed him great multitudes of people from Galilee, and from Decapolis, and from Jerusalem, and from Judea, and from beyond Jordan. (Matthew 4:25); Mark 5:2020And he departed, and began to publish in Decapolis how great things Jesus had done for him: and all men did marvel. (Mark 5:20); Mark 7:3131And again, departing from the coasts of Tyre and Sidon, he came unto the sea of Galilee, through the midst of the coasts of Decapolis. (Mark 7:31)). It was to Pella that the Christians fled just before the destruction of Jerusalem.

Strong’s Dictionary of Greek Words:

from 1176 and 4172; the ten-city region; the Decapolis, a district in Syria
KJV Usage:

Jackson’s Dictionary of Scripture Proper Names:

ten cities: the ten-city (region)

Potts’ Bible Proper Names:

Ten cities; region of ten cities:―a district in Assyria, Matt. 4:25. {Decem urbes}