The hypothesis of a special low-entropy boundary condition in the past, used in statistical accounts of the thermodynamic arrow of time. A complete account must also state its probability measure over compatible microstates and the macroscopic description used to define entropy.
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The "Past Hypothesis" is a concept from the philosophy of science, particularly in the context of thermodynamics and statistical mechanics. It proposes that the initial conditions of the universe were such that it began in a low-entropy state, which is crucial for explaining the arrow of time and the emergence of macroscopic phenomena. In simpler terms, the Past Hypothesis suggests that the universe started in a highly ordered state, with minimal disorder (or low entropy) at the Big Bang.