I'm reading a formula (lots of greek letters) and I see a symbol that looks like a backward six. That doesn't seem to be any greek letter... What is it?

Answer 1

You seem to be describing the nameless "partial derivative" symbol, sometimes called "del", a modified lower case Greek delta.

See
http://en.wikipedia.org/wiki/Partial_derivative

Answer 2

Ah ha! That's exactly what that is. And I knew that a long time ago. Thanks!

It doesn't show up on Wikipedia's greek alphabet page at all, but that's more than good enough! Thanks!