Something that is characteristic means something that tells you about the thing itself.
So if I told you something was see-through, you could narrow down the list of things that it might be, things that have that characteristic. You might think of glass, diamond, many things, but you wouldn't think of wood or brick, because they don't have the characteristic of being see-through.
You can measure light transmission, just like you can measure temperature, so they are both physical properties. However me telling you that something is at 60 degrees, doesn't narrow down the range of things much because there isn't anything that *by it's very nature* exists at 60 degrees in the same way that a diamond is always transparent.
Hope that helps.
