Easton's Bible Dictionary: M. G. Easton, M.A., D.D.
A foster-child, an Ephesian who accompanied Paul during a part of his third missionary journey (Acts 20:4, 21:29). He was with Paul in Jerusalem, and the Jews, supposing that the apostle had brought him with him into the temple, raised a tumult which resulted in Paul's imprisonment.
In writing to Timothy, the apostle says, "Trophimus have I left at Miletum sick" (2 Tim. 4:20). This must refer to some event not noticed in the Acts.