nullcline

A nullcline of x'=P(x,y),\ y'=Q(x,y) is a curve where one equation alone vanishes -- the x-nullcline is \{P=0\} (flow is vertical there), the y-nullcline is \{Q=0\} (flow is horizontal there); an equilibrium point is exactly where an x-nullcline and a y-nullcline cross.