A letter from the emperor Claudius to the city of Delphi.
It dates Gallio's proconsulship of Achaia to A.D. 51-52.
This helps establish the timeline of Acts, since Acts 18:12-17 mentions Gallio, toward the end of Paul's Corinthian stay.
(The ESV Study BibleTM, English Standard Version (ESV) by Crossway Bibles, 2007. Page 2301)