American Tract Society Bible Dictionary 
Is supposed to have been originally a province of Media, on its eastern side, which was raised into a distinct kingdom by Arsaces, B. C. 250. It soon extended itself over a great part of the ancient Persian Empire, and is frequently put for that empire in Scripture, and other ancient writings. Parthia maintained itself against all aggressors for nearly five hundred years, and was not subjugated even by the Romans; but in A. D. 226, one of the descendants of the ancient Persian kings united it to his empire, and Persia resumed it former name and dynasty.
The Parthians were celebrated, especially by the poets, for a peculiarity of their mode of fighting on horseback, which consisted in discharging their arrows while they fled. They would seem to have borne no very distant resemblance to the modern Cossacks. It is said the Parthians were either refugees or exiles from the Scythian nations. Jews and proselytes from among them were present at Jerusalem at the Pentecost, Acts 2:9 .
Kitto's Popular Cyclopedia of Biblial Literature 
Par´thia, the country of the Parthians, mentioned in , as being, with their neighbors the Medes and Elamites, present at Jerusalem on the day of Pentecost. The persons referred to were Jews from Parthia, and the passage is a strong evidence showing how widely spread were members of the Hebrew family in the first century of our era. The term originally referred to a small mountainous district lying to the northeast of Media. Afterwards it came to be applied to the great Parthian kingdom, into which this province expanded. Parthia Proper, or Ancient Parthia, lying between Aria and Hyrcania, the residence of a rude and poor tribe, and traversed by bare mountains, woods, and sandy steppes, formed a part of the great Persian monarchy, being a dependency on the satrapy of Hyrcania. Its inhabitants were of Scythian origin. They formed a part of the army of Xerxes, and were found in that of the last Darius. In the breaking up of the kingdom of Alexander the Parthians took sides with Eumenes, and became subject to Antigonus and the Seleucidæ. About 256 years before Christ Arsaces rose against the Syro-Macedonian power, and commenced a new dynasty in his own person, designated by the title of Arsacidæ. This was the beginning of the great Parthian Empire, which extended itself in the early days of Christianity over all the provinces of what had been the Persian kingdom, having the Euphrates for its western boundary, by which it was separated from the dominions of Rome. It was divided into eighteen provinces. Now at peace, now in bitter hostilities with Rome, now the victor and now the vanquished, the Parthians were never subjugated by the Romans. At length Artaxerxes founded a new dynasty. Representing himself as a descendant of the ancient Persian kings, and calling upon the Persians to recover their independence, he raised a large army, defeated the Parthians in a great battle, succeeded to all the dominions of the Parthian kings, and founded the new Persian Empire, to the rulers of which is commonly given the name of the Sassanidæ. The government of Parthia was monarchical; but as there was no settled and recognized line of succession, rival aspirants were constantly presenting themselves, which weakened the country with internal broils, especially as the Romans saw it to be their interest to foster dissensions and encourage rivalries, and led eventually to the overthrow of the dynasty in the case of the successful aspirant Artaxerxes. During the Syro-Macedonian period the Parthian and Jewish history kept apart in separate spheres, but under the Romans the Parthians defended the party of Antigonus against Hyrcanus, and even took and plundered Jerusalem.
The Nuttall Encyclopedia 
An ancient country corresponding to Northern Persia; was inhabited by a people of Scythian origin, who adopted the Aryan speech and manners, and subsequently yielded much to Greek influence; after being tributary successively to Assyria, Media, Persia, Alexander the Great, and Syria, they set up an independent kingdom in 250 B.C. In two great contests with Rome they made the empire respect their prowess; between 53 and 36 B.C. they defeated Crassus in Mesopotamia, conquered Syria and Palestine, and inflicted disaster on Mark Antony in Armenia; the renewal of hostilities by Trajan in A.D. 115 brought more varied fortunes, but they extorted a tribute of 50,000,000 denarii from the Emperor Macrinus in 218. Ctesiphon was their capital; the Euphrates lay between them and Rome; they were over thrown by Ardashir of Persia in 224. The Parthians were famous horse-archers, and in retreat shot their arrows backwards often with deadly effect on a pursuing enemy.