The Calm (ナギ節, Nagisetsu?) refers to an uncertain amount of time of peace in Spira between Sin's defeat at the hands of a summoner and its next public appearance in Final Fantasy X. In the long history of Sin terrorizing Spira, there have been four periods of time known as the Calm: 400 years ago Gandof's Calm, 230 years ago Ohalland's Calm, 100 years ago Yocun's Calm, and 10 years ago Braska's Calm. Yunalesca was the first to defeat Sin a thousand years ago, which quelled its rampage for a time, but the concept of Calm was not yet established and become doctrine in Yevon.
When a summoner sacrifices his or her life to defeat Sin via the Final Summoning, Yu Yevon possesses the posthumously titled high summoner's Final Aeon to create a new Sin as part of Spira's "spiral of death." As the pyreflies that composed the previous Sin have been scattered into the atmosphere, the new Sin built around the Final Aeon is still small and vulnerable, and thus goes into hiding. The duration of time of Sin's regeneration is christened the Calm, and the Calm Lands have long served as a place for summoners to call forth their Final Aeons. Yevon teaches that if Spira adheres to and follows its teachings, Sin may not return upon its next defeat.
This is, however, merely a way of appeasing the people by giving them hope of a brighter future. The Maesters of Yevon understand the true nature of Sin and the spiral of death it embodies: summoners are sacrificed to defeat Sin and if they succeed their aeon is possessed by Yu Yevon to create Sin anew.
While Yuna explains to Tidus what the Calm means to the people, she eventually learns the truth of the Final Summoning. She and her guardians look for another way to defeat Sin, and destroy Yu Yevon inside Sin. Yuna is hailed the high summoner of the Eternal Calm, but it was not to be, as Sin returns after three years, possibly beckoned back to Spira. Yuna plans to defeat it once more, even at the cost of her life.