In the context of the French Revolution, a Jacobin originally meant a member of the Jacobin Club (1789-1794). But even while the Club still existed, the name of Jacobins had been popularly applied to all promulgators of extreme revolutionary opinions "Jacobin democracy" for example is synonymous with totalitarian democracy.
Jacobinism is not related to Jacobitism or the English Jacobean period. In contemporary France this term refers to a centralistic conception of Republic, with a lot of power vested in the national government, at the expense of local governments.
In the sense of "promulgator of extreme revolutionary opinion", the word "Jacobin" passed beyond the borders of France and long survived the Revolution. Canning's paper, The Anti-Jacobin, directed against the English Radicals, consecrated its use in England; and in the correspondence of Metternich and other leaders of the repressive policy which followed the second fall of Napoleon in 1815, Jacobin is the term commonly applied to anyone with Liberal tendencies, even to so august a personage as the emperor Alexander I of Russia.
The English who supported the French Revolution during its early stages (or even throughout), were early known as Jacobins. These included the young Samuel Taylor Coleridge, William Wordsworth, and others prior to their disillusionment at the outbreak of The Terror. Others, such as William Hazlitt and Thomas Paine remained idealistic about the Revolution. Much detail on English Jacobinism is to be found in E. P. Thompson's The Making of the English Working Class.
The Anti-Jacobin was planned by George Canning when he was Under-Secretary of State for Foreign Affairs. He secured the collaboration of George Ellis, John Hookham Frere, William Gifford, and some others. William Gifford was appointed working editor. The first number appeared on November 20, 1797, with a notice that "the publication would be continued every Monday during the sitting of Parliament". A volume of the best pieces, entitled The Poetry of the Anti-Jacobin, was published in 1800. It is almost impossible to apportion accurately the various pieces to their respective authors, though more than one attempt has been made so to do.
Anger with the Jacobins inspired the Jesuit priest Abbe Augustin Barruel to create a vivid conspiracy theory about them.