  Open quantum and wave systems exhibit exotic degeneracies at exceptional
points in parameter space that have attracted considerable attention in various
fields of physics, including optics and photonics. One reason is the strong
response of open systems at such degeneracies to external perturbations and
excitations. We introduce two characteristics of exceptional points that
quantify the response in terms of energy eigenvalues and eigenstates,
intensity, and dynamics. The concept is verified for several physically
relevant examples. This work provides a new perspective on the physics of
exceptional points.
