In Final Fantasy X, the Calm is an unset amount of time of peace that follows when a summoner defeats Sin, dies and posthumously gains the title of High Summoner. Sin would inevitably return, at which point summoners would defeat Sin again, repeating the "spiral of death" indefinitely.
When Tidus first learns of this, he wonders why anyone would fight Sin if it always comes back, but Yuna convinces him that doing so is worth it. However throughout the story of Final Fantasy X, Yuna and her Guardians believe there must be another way to defeat Sin instead of continuing the 'spiral of death'. The Eternal Calm comes when Yuna is able to defeat Sin for good, bringing peace to Spira.