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.
In this sense the word 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 Tom Paine remained idealistic about the Revolution.
Do not confuse Jacobinism with Jacobitism (note the "t").
Some text here is taken from a 1911 encyclopedia. Please update as needed.