The Apostle Paul's second letter to the Thessalonians.
2 Thessalonians names Paul, Silas, and Timothy as its authors (same as 1 Thessalonians), and those who accept its authenticity generally conclude that it was written from Corinth shortly after 1 Thessalonians. (putting its composition circa AD 51)
A principal theme of 2 Thessalonians is that the parousia will not come immediately, suggesting that 1 Thessalonians' comments about the parousia may have been misunderstood.
The tone of this epistle is more of a stern lecture than the more positive 1 Thessalonians. Some take this to indicate Paul's frustration that the Thessalonians have not understood what he has been trying to teach them.
