can anyone can explain this notation to me?
That's just two symbols which don't make any sense like that.
Can you explain in which context you found them and/or maybe provide a copy of the relevant part of the document (like a scan or copying the sentence)?
If it were [tex]\nabla\times[/tex], then it would be the "curl" operator.
It's the gradient of the PI vector. lol.
Pi could be a http://en.wikipedia.org/wiki/Multiplication#Products_of_sequences" if it's followed by something
Separate names with a comma.