In plain words
The first part of the Bible, written before Jesus: the Law, the Prophets and the Writings, such as the Psalms.
The fuller definition
The first and longer part of the Bible, written mostly in Hebrew before the time of Jesus: the Law, the Prophets and the Writings, including the Psalms. The Scriptures Jesus himself read and prayed, which remain the Scriptures of the Jewish people.
In The Bible: Its books, its translations, and how Lutherans read it.