I've just started reading about the complexity of algorithms, but everywhere I look, it is only defined for one input $n$. For example an algorithm is cubic if its complexity is $O(n^3)$.
But what about when the complexity depends on several inputs? For example if an algorithm has complexity $O(n^2k)$, is it 'cubic', or maybe 'quadratic in $n$ and linear in $k$'?
I've also seen phrases such as 'cubic in $k$ and $n$'; what does this mean exactly?