I’ll put my money on (Disco) Stu Ball.
Precious
Woodpecker’s worse, I think.
14 k of g in a f p d?
Fnord
My father says that a silver reflective ball in the garden was in some places in the US referred to as a “kugel” or “garden kugel” when he was young. However, he says he never heard of it referring to a disco ball, which he thinks of as more like a mosaic thing.
He thought it was local slang.
I’ve heard it referred to as a witchball, for what it’s worth. Yes, I know it’s a compound word and no, I can’t guess at the etymology, but it’s all I’ve got.
Just how evil would it be for me to hang one of those in the living room?
Considering just how much fun the cats have with a simple laser pointer…
<where is the evil smiley when I really, really need one?>
What’s one word that means “disco ball”? “Discoball.”
Semprini.
[sub]No, I obviously don’t know, either.[/sub]
I used to sell those things and I have never heard a one word synonym for it that hasn’t been mentioned in this thread.
Although 90% I heard them called mirror balls and that’s how they were listed in our catalogue…I hardly ever heard anyone call them disco balls.
Splunge.
Glitzer.
How about glittersphere?
I think I made that up but probably not as there is really nothing original.
What can I say, it’s 3:31 and I can’t sleep. 
Dazzler?
I can’t believe this one only just now occurred to me, as it is absolutely perfect in every way:
spherpes
I think “llab” would also be appropriate. Sounds a bit awkward, though.
Just think what your could do with Star Ball! 
How about orb or globe?
Wikipedia discussion suggests bonspiel