The Great Disappointment in the Millerite movement was the reaction that followed Baptist preacher William Miller's proclamations that Jesus Christ would return to the Earth by 1844, which he called the Second Advent. His study of the Daniel 8 prophecy during the Second Great Awakening led him to conclude that Daniel's "cleansing of the sanctuary" was cleansing the world from sin when Christ would come, and he and many others prepared. When Jesus did not appear by October 22, 1844, Miller and his followers were disappointed.These events paved the way for the Adventists who formed the Seventh-day Adventist Church. They contended that what had happened on October 22 was not Jesus' return, as Miller had thought, but the start of Jesus' final work of atonement, the cleansing in the heavenly sanctuary, leading up to the Second Coming.

The Millerites were the followers of the teachings of William Miller, who in 1831 first shared publicly his belief that the Second Advent of Jesus Christ would occur in roughly the year 1843–1844. Coming during the Second Great Awakening, his beliefs were taken as predictions, spread widely, and believed by many, leading to the Great Disappointment.