5 September 1793. The term originates in the French Revolution, with the ascendancy of the Mountain (Jacobins) and the Committee of Public Safety, when Robespierre declared that Terror is the Order of the Day (full text of speech at link). With the fall of the Jacobins - known to history as 9 Thermidor, or the Thermidorian Reaction - the terms "terrorist" and "terrorism" took on the derogatory connotation we still associate with them.

Since then, numerous anti-state actors have been labeled as "terrorists" by various state entities. Gavrilo Princip, who murdered Franz Ferdinand in 1914, effectively starting World War I, as was labeled a terrorist, as was Sacha Ulyanov, who tried to assassinate Alexander III in 1887. Ulyanov, of course, was the older brother of another person who has been accused of terrorism, this time by using state power. He's better known under his alias, Vladimir Lenin.