∂u/∂t = D∇²u + u(1-u)(u-a) - v
∂v/∂t = ε(u - bv)
In excitable media (heart muscle, BZ reaction, neural tissue), a local stimulus triggers a traveling wave. When a wave breaks, the tip becomes a rotor — spiral wave. The tip trajectory (meander) depends on parameters: it can be circular, epicyclic (flower), or hypocyclic. Cardiac arrhythmias are attributed to spiral/scroll waves. The core (tip) is a phase singularity where all phases meet.