Phase space
The imaginary space of all the states a system can assume.
The phase space — also called state space — is the imagined region of all possible states a system can occupy, laid out along the dimensions that matter for that system.
The idea is simpler than it sounds. Take the quantities that describe a system — for a person, say, mood, tension and sleep — and make each one an axis. Every state the person is in at a given moment is then a single point in that space. As the state moves over time, the point traces a path: the trajectory. The phase space, in short, is the map; the trajectory is the route travelled across it.
Why this image helps
The value of the phase space is that it makes patterns visible which remain hidden in an ordinary time series. A system that keeps returning to the same region betrays an attractor; a system that suddenly jumps to another region, an order transition. What looks indistinguishable as separate measurements takes on a form in this spatial picture — a Gestalt, as synergetics would say.
For clinical practice this is more than a mathematical nicety. It invites us to see a patient not as a collection of separate symptom scores, but as a system moving through a space of possible states, inhabiting certain regions and avoiding others. The question then shifts from "how high is this score" to "where in their state space is this person, and in which direction are they moving".