Achaia is the southern part of Greece, where the cities of Athens and Corinth are located. It was conquered by Rome in 146 BC. Achaia and Macedonia (the northern part) are sometimes mentioned together when referring to the whole of Greece.
Gallio was proconsul of Achaia in AD 51-52, as noted in an inscription found at Delphi. This became an important date in establishing a chronology in the life and missionary work of Paul, as Paul was accused by the people of Achaia, before Gallio.
Paul traveled through Achaia on two of his missionary journeys. He spent much time there and expressed his love for the churches in Achaia, and two of Paul's letters are devoted to the Corinthians in Achaia.