Which term refers to space that is empty or void of molecules?

The term that refers to space that is empty or void of molecules is "Vacuum." In physical science, a vacuum is defined as a space entirely devoid of matter, meaning that there are no particles, including air molecules, present in that area. This condition can be achieved in a laboratory setting or can occur naturally in outer space, such as in the vast areas between celestial bodies where the density of particles is extremely low.

Interstellar space pertains specifically to the region between stars within a galaxy and, while it is indeed a vacuum in certain respects, it is not entirely void of matter as it can contain sparse hydrogen and helium atoms along with cosmic dust.

Microgravity describes a condition of very low gravity experienced in orbit, where free fall creates an environment where effects of gravity are not noticeable, but it does not imply a lack of molecules.

Ether is a historical term that referred to a hypothetical substance once thought to fill empty space, but it has no standing in modern physics and does not accurately describe a vacuum.

Thus, on the basis of the definition and characteristics of these terms, "Vacuum" is the correct answer as it most precisely captures the concept of a space completely free from molecules.