While most all of Christianity calls the first part of their Bible ''The Old Testament'', the Jewish people have never called it that. Why? Because that implies there is a ''New'' Testament - and to the Jewish people, there never was a ''New'' testament.
Additionally - the word ''Testament'' itself is a misnomer. 'Testament' is a Latin word used in English Bibles and the last time we checked, Jewish people do not speak Latin, nor did God ever write scripture in Latin.
The word God uses in scripture for this concept is ''Covenant.'' He makes ''Covenants'' with people.
So there is an ''Old'' and ''New'' Covenant that God made with Israel (see Jeremiah 31:31)
So why do the Jewish people call it the ''Tanakh'' (also spelled Tanach). Well, Tanakh is actually an acronym. Ta-na-kh (also spelled Tanach) is the three parts to this acronym. And that is because the Hebrew Bible is broken down into three parts....
''Ta'' is short for.... Torah (the first five books of the Hebrew Bible)
''Na'' is short for.... Navi'im (the major Prophets of the Hebrew Bible - like Isaiah)
''Kh'' is short for....Ketuvim (the Poetry books like Psalms)
So the Jewish people have spoken of their Bible like this for over two thousand years.
Even Jesus spoke of the Hebrew Bible this same way.
''Then he said to them, ''These are my words t hat I spoke to you while I was still with you—that everything written about me in the law of Moses, the prophets, and the psalms must be fulfilled.'' (Luke 24:44)
Did you see that? Jesus spoke about the Hebrew Bible as having three parts.
1) The Law of Moses (Torah)
2) The Prophets (Navi'im)
3) The Psalms (Ketuvim) - meaning Psalms and all the poetry books like Proverbs, Song of Solomon, etc.
Jesus spoke about the ''Torah, Navi'im and Ketuvim.''
He just spoke the long version. Today we use an acronym to save time.