Notation issue: Grad with a vector subscript

[Hoplite]

I'm reading a journal article at the moment which uses a piece of notation which they don't actually define. It looks like this:

$$\nabla_{\vec Q}$$

(As it happens, $$\vec Q$$ is an ordinary vector indicating the orientation of a polymer.)

I've never seen vector subscript on the gradient symbol "$$\nabla$$" before.

Could anyone please tell me what $$\nabla$$ with a vector subscript usually means?

 

[HallsofIvy]

First, since you mention polymers, I am inclined to think the symbol has more to do with chemistry than mathematics. If it really is purely mathematics then I would be inclined to think "the derivative in the direction of $\vec{Q}$".

 

[Hoplite]

Thanks, the derivative in the direction of the vector makes sense in context.