Adventism is a branch of Protestant Christianity that believes in the imminent Second Coming of Jesus Christ. It originated in the 1830s in the United States during the Second Great Awakening when Baptist preacher William Miller first publicly shared his belief that the Second Coming would occur at some point between 1843 and 1844. His followers became known as Millerites. After Miller's prophecies failed, the Millerite movement split up and was continued by a number of groups that held different doctrines from one another. These groups, stemming from a common Millerite ancestor, collectively became known as the Adventist movement.

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 teachings were spread widely and grew in popularity, which led to the event known as the Great Disappointment.