This is the original name of a narrow strip of territory in Greece, on the northwest of the Peloponnesus. Subsequently, it was applied by the Romans to the whole Peloponnesus, later called Morea, and the south of Greece. It was then one of the two provinces (Macedonia being the other) into which they divided the country when it fell under their dominion.
It is in this latter enlarged meaning that the name is always used in the New Testament (Acts 18:12, 27; 19:21; Romans 15:26; 16:5, etc.). It was at the time when Luke wrote the Acts of the Apostles under the proconsular form of government; hence the appropriate title given to Gallio as the “deputy,” i.e., proconsul, of Achaia (Acts 18:12).