See this question for some context about the stability of rotation of a body around different axes.
I am now trying to say that the rotation around the middle axis is very unstable, without using any formulae (since I am writing something like an abstract). Here is what I mean by "middle axis":
Let the moment of inertia around three principal axes be $I_1<I_2<I_3$. The axis with the moment of inertia $I_2$ is the "middle axis".
However, "middle axis" seems a vague phrase if it appears without some context, so can I really use it? What should I say instead? Is there any particular formal name for "middle axis"? I just feel that if I write "middle axis" without writing "the principal axis of inertia" first, people will struggle to see what I mean. But if I do include "the principal axis of inertia", it sounds redundant, since "middle axis" is certainly a principal axis.