Testament comes from the Latin testamentum which means an agreement or last will. When translated into the Greek diatheke and used in the Bible, it is often a synonym for “covenant.” This is the sense in which the Old Testament (and the New Testament) are to be understood.
The books of the Old Testament (the Torah, the Prophets and the Writings, according to their Jewish designation) give testimony to the covenant between God and Israel. The Old Testament and New Testament make up the Bible, often referred to as Holy or Sacred Scripture. The Jewish faith would refer to the books of our Old Testament as Hebrew Scripture.
There are 46 books in the Catholic and Eastern Orthodox Old Testaments, (73 in total including the New Testament) while the Protestant Bible contains only 39 Old Testament books. At the Protestant Revolution, seven books of the Old Testament were removed . . . seven books that have been included since the Bible was first collected together as acknowledged to be inspired by the Holy Spirit.
The seven Old Testament books missing in the Protestant Bible, but still found in the Catholic Bible, are the Books of Tobit, Judith, the Wisdom of Solomon, Sirach, Baruch, First Maccabees and Second Maccabees, plus portions of Esther and Daniel. The Protestant and Catholic New Testament books number the same.
The Old Testament was written by many different authors, mostly Hebrews, some unknown. These books of the Old Testament were written at different times over about 1,000 years; written at different places; and written for different purposes. (8:42)