Is this a sign for dot product?

Question

Whenever we write $\cos(\hat{\textbf{n}},z)<0$ do we mean that the angle between the unit normal and the $z$ axis is greater than 90$^\circ$?

Or is it some sort of reference to the dot product? Context:

Answer 1

It's both :)

I believe $(\hat{\textbf{n}},z)$ stands for angle between unit vector $\vec{n}$ and unit vector of $z$ axis, usually denoted with $\vec k$.

So:

$$cos(\hat{\textbf{n}},z)<0$$

...is equivalent to:

$$\angle(\vec n, \vec k)>90^\circ$$

...or:

$$\vec n\cdot\vec k<0$$