= Solution
The <autonomous planar system> has two real state variables, so its <phase space> is a subset of $\mathbb R^2$. It is autonomous because its <vector field> $(X(x,y),Y(x,y))$ has no explicit time dependence. The <nullclines> are $X(x,y)=0$ and $Y(x,y)=0$, on which the respective coordinate's instantaneous rate of change vanishes. A <critical point of a dynamical system> is an intersection where \b[$X=Y=0$]; the constant trajectory at that point is an <equilibrium of an autonomous differential equation>. A nullcline alone need not consist of equilibria.
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