How the French Revolution invented the terrorist.

The noun terrorist, first recorded in November 1794, is from French terroriste, first attested in September of the same year, shortly after the execution of Maximilien de Robespierre on 28^th July. These words were used to designate an adherent or supporter of his faction, the Jacobins, who, from about March 1793 to July 1794, advocated and practised the Terror, that is to say, methods of partisan repression and bloodshed in the propagation of the principles of democracy and equality.

In a speech to the Convention, delivered on 5^th February 1794, Robespierre, who was the leader of the Comité de salut public (Committee of Public Safety), justified the Terror:

We must smother the internal and external enemies of the Republic or perish with it; now in this situation, the first maxim of your policy ought to be to lead the people by reason and the people’s enemies by terror.
If the spring of popular government in time of peace is virtue, the springs of popular government in revolution are at once virtue and terror: virtue, without which terror is fatal; terror, without which virtue is powerless. Terror is nothing other than justice, prompt, severe, inflexible; it is therefore an emanation of virtue; it is not so much a special principle as it is a consequence of the general principle of democracy applied to our country’s most urgent needs.
It has been said that terror is the principle of despotic government. Does your government therefore resemble despotism? Yes, as the sword that gleams in the hands of the heroes of liberty resembles that with which the henchmen of tyranny are armed. Let the despot govern by terror his brutalised subjects; he is right, as a despot. Subdue by terror the enemies of liberty, and you will be right, as founders of the Republic. The government of the revolution is liberty’s despotism against tyranny.
     original text:
Il faut étouffer les ennemis intérieurs et extérieurs de la république, ou périr avec elle ; or, dans cette situation, la première maxime de votre politique doit être que l’on conduit le peuple par la raison, et les ennemis du peuple par la terreur.
Si le ressort du gouvernement populaire dans la paix est la vertu, le ressort du gouvernement populaire en révolution est à la fois la vertu et la terreur : la vertu, sans laquelle la terreur est funeste ; la terreur, sans laquelle la vertu est impuissante. La terreur n’est autre chose que la justice prompte, sévère, inflexible ; elle est donc une émanation de la vertu ; elle est moins un principe particulier qu’une conséquence du principe de la démocratie appliqué aux plus pressants besoins de la patrie.
On a dit que la terreur était le ressort du gouvernement despotique. Le vôtre ressemble-t-il donc au despotisme ? Oui, comme le glaive qui brille dans les mains des héros de la liberté ressemble à celui dont les satellites de la tyrannie sont armés. Que le despote gouverne par la terreur ses sujets abrutis, il a raison comme despote : domptez par la terreur les ennemis de la liberté, et vous aurez raison comme fondateurs de la république. Le gouvernement de la révolution est le despotisme de la liberté contre la tyrannie.

“It has been said that terror is the principle of despotic government” refers to the following from L’Esprit des lois (The Spirit of Laws – 1748), by the French political philosopher Charles Louis de Secondat, Baron de Montesquieu (1689-1755):

Of the Severity of Punishments in different Governments.
The severity of punishments is fitter for the despotic government, whose principle is terror, than for the monarchy or the republic, whose spring is honour and virtue.
     original text:
De la sévérité des peines dans les divers gouvernements.
La sévérité des peines convient mieux au gouvernement despotique, dont le principe est la terreur, qu’à la monarchie et à la république, qui ont pour ressort l’honneur et la vertu.

The first known user of the word terroriste was the political journalist and agitator ‘Gracchus’ Babeuf (François-Noël Babeuf – 1760-97) in Journal de la Liberté de la Presse of 11^th September 1794 (25 fructidor, l’an 2ème de la République):

L’Orateur du Peuple, l’Ami des Citoyens, periodicals constantly appreciated by the patriots of the good old days, that is to say, of the years one, two, three and four of liberty*, but which cannot be guaranteed to appeal to the terrorist patriots (the French always love variety, this expression will soon come into fashion) which cannot be guaranteed, say I, to appeal to the terrorist patriots of the year two of the republic*; these two newspapers are back in print next to mine.
     original text:
L’Orateur du Peuple, l’Ami des Citoyens, ouvrages périodiques constamment goûtés par les patriotes du bon vieux tems [sic], c’est-à-dire, des ans premier, deux, trois et quatre de la liberté, mais qu’on ne répond pas qui plairont aujourd’hui aux patriotes terroristes (les Français aiment toujours la variété, cette expression va venir à la mode) qu’on ne répond pas, dis-je, qui plairont aux patriotes terroristes de l’an deux de la république ; ces deux journaux reparoissent [sic] à côté du mien.

The earliest instance of the English word terrorist that I have found is from The Dublin Evening Post (Dublin, Ireland) of 15^th November 1794:

21 VENDEMIAIRE—OCT. 12.
Twenty sections of Paris appeared in a mass at the bar, for the purpose of disavowing the petitions presented in their name by a few intriguing men, who met at midnight, and who pretended be the organs of the wishes of the sections.— They declared that their wish was that the anarchists and continuators of the system of Robespierre should be destroyed. Their sentiments, they said, were similar to these contained in the Address to the National Convention.
The Section of Lepelletier, formerly the firmest support of the Jacobin Society, declared eternal war against the terrorists and intriguers. They expressed their aversion to those men of blood, and to the impure horde of self-interested men.

* The “era” of liberty started in 1789, when the Revolution began; but the republican calendar started on 22^nd September 1792, when the Republic was proclaimed.