From Webster:
Definition of divergence
1a
: a drawing apart (as of lines extending from a common center)
b
: difference,
disagreement
c evolutionary biology
: the development of dissimilar traits or features (as of body structure or behaviour) in closely related populations, species, or lineages of common ancestry that typically occupy dissimilar environments or ecological niches
: divergent evolution
2
: a deviation from a course or standard
3
: the condition of being mathematically
divergent
I would say 2 applies here. Yaw divergence would be that the tail rotor doesn't do, what it is supposed to do.