The Epistle to the Laodiceans is a possible lost letter of Paul, the original existence of which is inferred from an instruction to the church in Colossae to send their letter to the church in Laodicea, and likewise obtain a copy of the letter "from Laodicea" (Greek ek laodikeas ἐκ Λαοδικείας).
And when this epistle is read among you, cause that it be read also in the church of the Laodiceans; and that ye likewise read the from Laodicea.
— Colossians 4:16 KJV
Several ancient texts purporting to be the missing "Epistle to the Laodiceans" have been known to have existed, most of which are now lost. These were generally considered, both at the time and by modern scholarship, to be attempts to supply a forged copy of a lost document. The exception is a Latin "Epistle to the Laodiceans," which is actually a short compilation of verses from other Pauline epistles, principally Philippians, and on which scholarly opinion is divided as to whether it is the lost Marcionite forgery or alternatively an orthodox replacement of the Marcionite text. In either case it is generally considered a "clumsy forgery" and an attempt to seek to fill the 'gap' suggested by Colossians 4:16.
Some ancient sources, such as Hippolytus, and some modern scholars consider that the epistle "from Laodicea" was never a lost epistle, but simply Paul recycling one of his other letters (the most common candidate is the contemporary Letter to the Ephesians), just as he asks for the copying and forwarding of the Letter to Colossians to Laodicea.
Read more about Epistle To The Laodiceans: Letter To The Laodiceans, The Colossians 4:16 Mention, Identification With The Epistle To The Ephesians, The Marcionist Epistle To The Laodiceans, The Latin Vulgate Epistle To The Laodiceans