The story
The concept was developed in the late 19th century by Ludwig Boltzmann, Henri Poincaré and Josiah Willard Gibbs.
Etymology from Wiktionary, CC BY-SA 4.0.
phase space
Today it means: Given a (dynamical) system, any topological space such that every point in the space's underlying set uniquely represents a state of the system and every possible state is represented by some point; Full definition →