And how does it (or does not) correspond to "polymorphic"? Occasionally I see this notion like in: Implement and represent polyadic operations. I checked Wiktionary but it only gives a general meaning, not specific to functional programming.
It's really simple to understand.
How does this relate to polymorphy? Imho it does not relate to polymorphy at all. I think often times polyadic and variadic are substitutable. See wikipedia for further information.
How does this relate to functional programming?
I think the following wikipedia quote sais it all: