What is "unitarity"

n. The quality or state of being unitary.

In quantum physics, unitarity is a restriction on the allowed evolution of quantum systems that ensures the sum of probabilities of all possible outcomes of any event is always 1.

Since unitarity of a theory is necessary for its consistency (it is a very natural assumption, although recently questioned), the term is sometimes also used as a synonym for consistency, and is sometimes used for other necessary conditions for consistency, especially the condition that the Hamiltonian is bounded from below. This means that there is a state of minimal energy (called the ground state or vacuum state). This is needed for the second law of thermodynamics to hold.