Abolitionism was the organised campaign to end the transatlantic slave trade and then slavery itself, first built into a sustained political movement in Britain and the United States in the closing decades of the eighteenth century, well after individual moral objection to slavery had existed for far longer without producing a movement able to act on it.
Britain moved first. The Society for Effecting the Abolition of the Slave Trade, founded in 1787, combined religious conviction, chiefly among Quakers, with a sustained parliamentary campaign led by William Wilberforce, and Parliament banned the trade itself in the Slave Trade Act of 1807. Slavery in most of the British Empire followed in 1833, with the Slavery Abolition Act. In the United States the movement grew more slowly and far more violently contested, built around figures such as William Lloyd Garrison, whose newspaper The Liberator began publishing in 1831, and Frederick Douglass, an escaped slave who became one of the movement's most forceful public voices; American slavery ended only with the Civil War and the Thirteenth Amendment, ratified in 1865.
The movement combined moral persuasion, political lobbying and direct action, including networks that helped enslaved people escape, and its methods and its central argument, that a legal status could be morally illegitimate regardless of what the law said, became a template later invoked by suffrage and civil rights movements that followed it by generations.