A means of describing fully the state, or evolution of the state, of a system with time. Time may pass discretely, in which case the system evolves according to a recurrence relation, or may pass continuously, in which case the system evolves according to a differential equation. If the system has n degrees of freedom, the phase space of possible states is an n-dimensional manifold. For example, with a double pendulum this space would be a torus, as the space is entirely determined by the two angles the pendula make with the downward vertical.