From BiblePortal Wikipedia

Hastings' Dictionary of the Bible [1]

COS . An island off the coast of Caria, S.W. of Asia Minor, famous for its fertility and beauty. It was a Dorian colony, and a great seat of the worship of Æsculapius and of the study of medicine. Its position made it also an important place from a trade point of view, as it lay on the cross lines of traffic between Greece, Asia Minor, Syria, and Egypt. It is uncertain whether Cos, which had been a faithful ally of the Romans, was incorporated in the province of Asia in b.c. 139 (see Caria), but it certainly was a part of it in the time of Augustus. Its trade connexion made it one of the Jewish centres of the Ægæan. The Jews there were favoured by the Romans in b.c. 139 138 ( 1Ma 15:23 ). It was a place on the route of the Jewish pilgrims to Jerusalem (cf.   Acts 21:1 ). Herod the Great was a benefactor of the people of Cos.

A. Souter.

People's Dictionary of the Bible [2]

Cos ( Kŏs ) or Coos ( Kô-Os ). A small island in the Ægean sea off the coast of Caria, the birthplace of Hippocrates, with a chief town of the same name, in which was a famous temple of Æsculapius. The island was celebrated for its wines, beautiful stuffs, and ointments. Paul passed a night here on his voyage from Miletus to Judea.  Acts 21:1.

Smith's Bible Dictionary [3]

Cos. (Now Stanchio or Stanko ). This small island of the Grecian Archipelago has several interesting points of connection with the Jews. Herod the Great conferred many favors on the island. St. Paul, on the return from his third missionary journey, passed the night here, after sailing from Miletus. Probably referred to in  Acts 21:1.

Fausset's Bible Dictionary [4]

Cos or Coos: now Stancho , a contraction of Eis Teen Cηoa . Paul passed the night on this island on his way by sea from Miletus to Rhodes ( Acts 21:1). It is N.W. of Rhodes; 25 miles long by 10 miles wide. The chief town was on the N.E. of the island, near the promontory Scandarium.

Holman Bible Dictionary [5]

 Acts 21:1

International Standard Bible Encyclopedia [6]

kos ( Κῶς , Kō̇s , "summit"; the King James Version Coos ): An island off the coast of Caria, Asia Minor, one of the Sporades, mountainous in the southern half, with ridges extending to a height of 2,500 ft.; identified with the modern Stanchio. It was famous in antiquity for excellent wine, amphorae, wheat, ointments, silk and other clothing ( Coae vestes ). The capital was also called Cos. It possessed a famous hospital and medical school, and was the birthplace of Hippocrates (the father of medicine), of Ptolemy Philadelphus, and of the celebrated painter Apelles. The large plane tree in the center of the town (over 2,000 years old) is called "the tree of Hippocrates" to this day. The older capital, Astypalaea, was in the western part of the island, the later (since 366 bc) in the eastern part. From almost every point can be seen beautiful landscapes and picturesque views of sea and land and mountain.

Cos was one of the six Dorian colonies. It soon became a flourishing place of commerce and industry; later, like Corinth, it was one of the Jewish centers of the Aegean, as well as one of the financial centers of the commercial world in the eastern Mediterranean. Among the benefactors of the people of Cos was Herod the Great. It is mentioned in connection with Paul's third missionary journey in  Acts 21:1 , and in its relations with the Jews in 1 Macc 15:23; Ant , Xiv , vii, 2; x, 15; BJ , I, xxi, 11. For a list of works on the island see Paton-Hicks, Inscriptions of Cos , ix.

The Nuttall Encyclopedia [7]

An island in the Ægean Sea, birthplace of Hippocrates and Apelles.