Before revision: Hindsight rationality is an approach to playing multi-agent, general-sum games that prescribes no-regret learning dynamics and describes jointly rational behavior  with mediated equilibria.
After revision: Hindsight rationality is an approach to playing multi-agent, general-sum games that prescribes no-regret learning dynamics for individual agents with respect to a set of deviations, and further describes jointly rational behavior with mediated equilibria.