Definition:
This is a local indicator that assesses whether the system's dynamics are compatible with the constraints at a given time.
Simplified formula:
$$
F(x) \cap T_K(x) \neq \emptyset
$$
with TK(x) = tangent cone to the set of constraints K at x, and F(x) = system dynamics.
Usefulness:
If the intersection is empty, then the state is not viable in the short term.