It's not a song. It's not a suite. It's not a concept album.
And yet it's also, somehow, all three.
I don't particularly have issue with people referring to Six Degrees as a single song. It is. What does bother me, though, is when people "correct" you when you refer to, say, Solitary Shell as a song.
It is a song. It was released as its own single. It has its own unique chorus, and a coherent structure in of itself. They play it live on its own. They even indexed it separately so that listeners can choose to consume it in their own way. Solitary Shell is a statement unto itself. Its own musical journey. It's absolutely fair to refer to it as a song.
But, frankly, we're trying to apply specific terminology to ambiguous concepts. This will never end well. They're all just things. "Statements of sound," if we want to be politically correct. And there's no reason they have to fit under only one umbrella.
You can call them what you like. Losing Time is a "Gerald." Six Degrees is a "slide." Geralds and slides, by the way, are somehow simultaneously both completely distinct from each other, and exactly the same.
The band said it was all to be seen as one song, but they also indexed the movements so we can choose to consume it whatever way we like, play ATCR/Losing Time live as a single unit, and released The Test That Stumped Them All as an individual song on their best of CD. The fans are way more anal about the terminology than the band ever have been or ever will be.
Six Degrees is a song if you listen to it all in a row. Solitary Shell is a song when they play it live. It's just a matter of context.
(Incidentally, I'd say "suite" is the probably the definition that most faithfully represents both sides of the argument. But hey, we've already established that it's a Gerald. Or was it a parsnip?)