The type "atom"[1], is described in "Core Language" as follows:
> An atom can hold a single floating point numeric value, or an integer
I don't know why it isn't called "number", but it exists.
The page on atom also mentions:
> It can also hold a raw pointer, such as allocated memory or a call_back address, but that is typically only used when interfacing to external code in a .dll or .so file.