For a function with 2 degrees of freedom, the 2-dimensional phase space that is accessible to the function or object is called its phase plane.
Phase Plane
See also
Phase Space, World LineExplore with Wolfram|Alpha
References
Tabor, M. "Dynamics in the Phase Plane." §1.3 in Chaos and Integrability in Nonlinear Dynamics: An Introduction. New York: Wiley, pp. 13-20, 1989.Referenced on Wolfram|Alpha
Phase PlaneCite this as:
Weisstein, Eric W. "Phase Plane." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/PhasePlane.html