B Visible light and light are two different things?

This comes and goes with the word "usually". Usually, light is the visible part of the spectrum, perhaps extended to near IR and UV frequencies. Since they all are electromagnetic radiation, light and visible light are basically the same. EM waves with near-light frequencies are sometimes referred to as IR light or UV-A/B/C light, but this is hair splitting. In general light is visible and other parts of the spectrum have different names, like radiowaves, X-rays, gamma-rays and so on.

 

Note the term "speed of light"...

 

As it's all electromagnetic radiation you can call it as such, our for the lazy ones: light. But this is unusual. You don't come home from the dentist and tell he has lightened you if he took an X-ray of your teeth, don't you?

 

Because natural language is not a completely precise way of stating ideas. It may be usual to restrict the word "light" to the visible part of the electromagnetic spectrum, but not everyone is going to consistently do what's usual. It often doesn't matter, but when it does you may have to determine from the context what is intended.

The relationship between photons and electromagnetic radiation is much more complicated than any popular source would suggest. Generally it is unnecessary and even confusing to use the concept of photons in any discussion not involving quantum mechanical phenomena; but if you want to learn more you might try this thread and especially the links in the first two posts: https://www.physicsforums.com/threads/what-is-a-photon.879128/#post-5522356

 

Perfect! This is number two on the list of properties needed to improve knowledge: skepticism! (Number one is curiosity.)

One can call it light because it is shorter and in the end the same thing, only different by wavelength, resp. frequency. As in the case of NASA

We may not forget their goal. Here common people are addressed and it's probably the shortest way to explain what EM radiation really is: visible and invisible light. It's not meant to be scientifically rigorous. How would you explain microwaves, X-rays, radio signals, IR radiation and so on and so on if it has to be brief, understandable to everyone and a side note, because the purpose of the webpage is a different one, in this case astronomical observations and conclusions. The alternative would be either a large detour or not easy to understand and in any case: longer. It is as wrong as saying an ultrasound examination is by sound or elephants' communication is by sound (infrasound), dog whistles uses sound (although neither can be heard by us) and similar. They all are a kind of sound, although they don't make a sound. That's the same with light. All the same electromagnetic radiation: some is visible to us, why we usually call it light, some is visible to bees, although we don't, with some we heat our meals, open car doors and switch tvs and others we use for medical purposes. It's up to everybody to make that distinction (visible=light - invisible=rest of the spectrum) or not. If you don't, chances are high to be misunderstood. But it can abbreviate a longer course which is done on the NASA page, just because they do not want to hold a lecture on EM radiation there. They even start by telling it's the same but eventually invisible!

 

It's back now.

 

As with most words, context is important. Light will mean visible light, IR light, UV light, or even most or all of the EM spectrum depending on how it's used. All of these uses are perfectly legitimate in most cases.