ACAYA
Originally a Greek state north of the Peloponnese (modern Morea); included Corinth and its isthmus; In NT times, all of Greece was divided between Macedonia and Achaia, under Roman administration; Achaia was a senatorial province;
However, in the poets, Achaia frequently designated the entirety of Greece. Tiberius united the two districts into one imperial province under procurators; but Claudius returned Achaia to the Senate under a proconsul.
Thus, Luke was correct in mentioning Gallio as proconsul of Achaia (Acts 18:12). The NT uses this designation in the sense of the Roman administration. Epenetus was the first convert from Achaia (Rom. 16:5); Stephanas, among the first (1 Cor. 16:15). (See also Acts 18:27; 19:21; Rom. 15:26; 2 Cor. 1:1; 9:2; 11:10; 1 Thes. 1:7, 8.)