The name originally of a narrow strip of territory in Greece, on the north-west of the Peloponnesus. Subsequently it was applied by the Romans to the whole Peloponnesus, now called the 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; Rom. 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).
Achaia
Achaia is a region Paul keeps coming back to, usually in the same breath as Macedonia. In Acts, Paul plans to pass through both on the way to Jerusalem. In his letters, he calls the household of Stephanas “the firstfruits of Achaia”, the first to come to Christ there. And in Romans he mentions that the churches of Macedonia and Achaia were glad “to make a certain contribution for the poor saints which are at Jerusalem.” Paul also tells the Corinthians their zeal “hath provoked very many.” Romans 15:26 is a short place to start.
6 verses
Acts
After these things were ended, Paul purposed in the spirit, when he had passed through Macedonia and Achaia, to go to Jerusalem, saying, After I have been there, I must also see Rome.
Romans
For it hath pleased them of Macedonia and Achaia to make a certain contribution for the poor saints which are at Jerusalem.
Likewise greet the church that is in their house. Salute my wellbeloved Epaenetus, who is the firstfruits of Achaia unto Christ.
1 Corinthians
I beseech you, brethren,(ye know the house of Stephanas, that it is the firstfruits of Achaia, and that they have addicted themselves to the ministry of the saints,)
2 Corinthians
For I know the forwardness of your mind, for which I boast of you to them of Macedonia, that Achaia was ready a year ago; and your zeal hath provoked very many.
As the truth of Christ is in me, no man shall stop me of this boasting in the regions of Achaia.
From Easton's Bible Dictionary
Matthew George Easton, 1897