I cannot remember where but I recall seeing something somewhere calling the Pi an 'Embedded Linux'. What does that mean? Because I believe it can run other OSes other than Linux such as the Windows 10 IOT core even though Linux is the most common. Did they have the facts wrong or am I just confused?
The Pi isn't really a "linux" anything, it just so happens that GNU/Linux, more specifically Raspbian, which is more-or-less Debian with some tweaks (and also a few pieces of pi specific software -- the Foundation seems to have a dev that puts some work into adding stuff to the GUI, etc, as demonstrated by some recent additions -- but there are some independent things as well, such as
omxplayer) has come to predominate amongst the userbase, with the encouragement of the designers/manufacturers.
There are a number of reasons this worked out that way:
GNU/Linux is open source, meaning both freely adaptable and redistributable by third parties (in fact, all the binary distros are from third parties such as Debian -- "GNU" and "Linux" are very separate organizations).
The linux kernel has probably been adapted to a wider range of contexts than any OS kernel in history, and would have required relatively little work to get going on the Pi's SoC ("relative" to writing something brand new, which would be infeasible -- I've seen cost estimates about how much it might cost to develop the kernel from scratch and it is into the billions of dollars). I think there are some other potential candidates, but not with the same amount of playtesting, including the long paired userland (the "GNU" half of things).
However, while I am sure the Pi designers were aware of linux, I think it would be pushing things a bit to say it was actually designed for it.
Simply refers to a computer system dedicated to a special purpose, where the computer is not really a defining focal point of the "thing" as a whole (although it is probably essential to it functioning). This covers everything from your set-top cable box to the electronic brains in cars, military weapons systems, etc. They may be small and simple or large and complex. The linux kernel (as distinct from the aforementioned userland paired with it in general purpose systems) had a well established place in parts of this realm already (including those set-top boxes, and home routers, and no doubt all kinds of other things like POS systems, etc.).
The Pi all by itself isn't really an "embedded" device in this sense, as it has a general purpose set of peripheral interfaces and was not designed with any specific purpose in mind. But its small form factor and low power and cost make it a good candidate for all sorts of embedded scenarios. This is why a more appropriate categorization is "dev(elopment) board". It is an ideal platform for prototyping small scale embedded systems.