The interstellar matter that will eventually form a star is called a molecular cloud.

Here's why:

* Molecular: These clouds are primarily composed of molecules, primarily hydrogen and helium, but also containing small amounts of heavier elements like carbon, oxygen, and nitrogen.

* Cloud: These vast, cold, and dense regions of space are where stars are born.

So, "molecular cloud" is the most accurate and widely used term for the interstellar matter that becomes a star.