a message on paper from one person or group to another
G1992 epistolē – a letter, epistle
Acts 15:30 So, when they were sent off, they came to Antioch. Having gathered the multitude together, they delivered the letter.
1 Corinthians 5:9 I wrote to you in my letter to have no company with sexual sinners;
Colossians 4:16 When this letter has been read among you, cause it to be read also in the assembly of the Laodiceans; and that you also read the letter from Laodicea.
2 Peter 3:15-16 Regard the patience of our Lord as salvation; even as our beloved brother Paul also, according to the wisdom given to him, wrote to you; 16 as also in all of his letters, speaking in them of these things. In those, there are some things that are hard to understand, which the ignorant and unsettled twist, as they also do to the other Scriptures, to their own destruction.
There is a clear distinction between a ‘letter’ and ‘scripture.’ The majority of the New Testament is not scripture (inspired by YHWH) but is a collection of letters written by men such as Paul and the other apostles. The verse in 2 Peter does a good job of showing the distinction between the two. Peter points out that Paul’s ‘letters’ can be hard to understand and that people who do not have a solid understating of the ‘scriptures’ might twist what Paul is really trying to say to their own destruction.