In order to study the sticky wicket that is terrorism, the first thing one must understand is that even scholars don't have a singular, unified definition of what terrorism actually is. The closest and most commonly accepted (or at least used as a framework for other definitions) is [Alex P. Schmid's revised definition from 2011] (http://www.terrorismanalysts.com/pt/index.php/pot/article/view/schmid-terrorism-definition/html). It's rather multi-faceted, but the main components to keep in mind are these:

Terrorism refers, on the one hand, to a doctrine about the presumed effectiveness of a special form or tactic of fear-generating, coercive political violence and, on the other hand, to a conspiratorial practice of calculated, demonstrative, direct violent action without legal or moral restraints, targeting mainly civilians and non-combatants, performed for its propagandistic and psychological effects on various audiences and conflict parties; ...

Terrorism as a tactic is employed in three main contexts: (i) illegal state repression, (ii) propagandistic agitation by non-state actors in times of peace or outside zones of conflict and (iii) as an illicit tactic of irregular warfare employed by state- and non-state actors;

So, by this definition, you can see that at its core, terrorism as an act is inherently political. Although Schmid allows for state actors to engage in terrorism, in the circles in which I've studied it, other scholars generally agree that in order for something to be terrorism, it must not come from an established state entity (no one called Nazis terrorists, for instance, because they were the ruling party in Germany).

So, to answer your question, the guerrilla tactics that have evolved to the modern day concept of terrorism, as nebulous as it is, came from very specific political movements in various parts of the world. It was always meant to be a political term, not a description of the fear response caused by terrorist acts. In their book Friction: How Radicalization Happens to Them and Us, psychologists and terrorism scholars Clark McCauley and Sophia Moskalenko credit the 19th century Russian organization The People's Will, which would be responsible for the assassination of Tsar Alexander II, as "the first modern terrorist group"(5). They specify that prominent member Andrei Zhelyabov advocated for violence as a means to an end, using the term "terror":

The lack of political will among the peasants and their fear of persecution inspired Zhelyabov along with some members of Land and Freedom [the party that preceded People's Will] to propose political terrorism as the only way to help the peasants. They believed that, if the czar and his family were eliminated, peasants would join with the students in demanding democracy and adequate land endowment from the government.

Zhelyabov advocated "relentless, unending political terror" as the only way to crack the monolith of the monarchy and open a possibility of social change. (52)

Interestingly, although this seems to be the origin of the developed word "terrorism," the term was not in common usage in America until well into the 20th century. If non-state actors were causing a ruckus, generally they were called "anarchists," such as [the assassin of President McKinley in 1901] (http://fineartamerica.com/featured/1-mckinley-assassination-granger.html). "Terrorist" didn't become in vogue in the United States until the 1970s, but usually to refer to the radical leftists such as the Weather Underground rather than the most common usage we see today.

At the risk of getting too close to current events than this sub allows, I will throw in that the conflation with "terror" the emotion rather than "terrorism" the political tactic didn't really arise until President Bush declared a war on terror rather than terrorism... a campaign I guarantee you will make every terrorism scholar cringe when they hear its name due to the the syntax error.

...I hope this helps! I nearly leapt out of bed in joy when I finally came across a question I was equipped to answer. :)