Paul, Silvanus, and Timothy, to the assembly of the Thessalonians in God our Father, and the Lord Jesus Christ:
A Thessalonian was one who was from Thessalonica, just as people from America are called Americans (Acts 20:4; Acts 27:2; 1 Thessalonians 1:1; 2 Thessalonians 1:1). The city of Thessalonica is "a city on the Thermaic Gulf, now called the Gulf of Salonika" (Westminster Dictionary of the Bible 941).