BATNA is an acronym that stands for "Best Alternative To a Negotiated Agreement." It is a key concept in negotiation theory, first introduced by Roger Fisher and William Ury in their book "Getting to Yes." It represents the most advantageous course of action a party can take if negotiations fail and an agreement cannot be reached. Understanding one's BATNA is crucial for a negotiator because it provides a benchmark against which any proposed agreement can be measured. It helps a party decide whether to accept a deal or walk away.