(ə-trāk'tər) A set of states of a dynamic physical system toward which that system tends to evolve, regardless of the starting conditions of the system. ◇ A point attractor is an attractor consisting of a single state. For example, a marble rolling in a smooth, rounded bowl will always come to rest at the lowest point, in the bottom center of the bowl; the final state of position and motionlessness is a point attractor. ◇ A periodic attractor is an attractor consisting of a finite or infinite set of states, where the evolution of the system results in moving cyclically through each state. The ideal orbit of a planet around a star is a periodic attractor, as are periodic oscillations. A periodic attractor is also called a limit-cycle. ◇ A strange attractor is an attractor for which the evolution through the set of possible physical states is nonperiodic (chaotic), resulting in an evolution through a set of states defining a fractal set. Most real physical systems (including the actual orbits of planets) involve strange attractors.