In plain words
The book of writings Christians receive as God’s word: the Old Testament and the New Testament.
The fuller definition
The collection of writings Christians receive as God’s word: the Old Testament, written before Jesus, and the New Testament, written by his first followers. Sixty-six books in all, written over many centuries in Hebrew, Aramaic and Greek. Lutherans read the whole of it in the light of Christ.
Where it comes from
2 Timothy 3:16
In The Bible: Its books, its translations, and how Lutherans read it.