Here's a breakdown of why:

* Medium: A medium is any substance that light can pass through. It can be a solid, liquid, or gas.

* Examples: Air, water, glass, and even a vacuum are all mediums through which light can travel.

* Speed: The speed of light changes depending on the medium. It travels fastest in a vacuum (like space) and slower in denser materials like water or glass.

So, while "air" and "water" are examples of mediums, the general term for the material light travels through is medium.