Terrorism is a difficult to define, and has no accepted definition in the international community due to the political difficulty of deeming one group freedom fighters and another terrorists. Most cultures have a group or groups in their history which could conceivably be regarded as terrorists under reasonable definitions, so it's hard to reach a consensus that pleases everyone without condemning one group or another. Were the American colonists, the Israeli Zionist movement, or Hezbollah terrorists? It depends on who you ask and how the term is defined.

I'll use the commonly accepted definition of terrorism as the systematic use of violence for the purposes of political coercion through the manipulation of public opinion using fear. John Brown fits this description accurately. He was convinced that there was no peaceful means of ridding the US of slavery, and wanted to violently overthrow the established order in order to achieve his political objectives (Eric Foner's Forever Free outlines his conviction that his actions were divine retribution for the sin of slavery). Brown spoke of achieving "shock" and "terror" through his actions when other tried to persuade him to continue pursuing peaceful means of political change, and had no qualms about killing civilians in cold blood in order to achieve these ends.

I took a class on John Brown last semester and while I don't love answering this question I can give you some solid sources on Brown. David S. Reynold's biography is excellent, and a great place to start if you haven't read much about Brown. Reynolds is iffy on the issue of calling Brown a terrorist and I would give you a quote right now but i don't have my copy with me at the moment. I would also recommend a primary source collection, Zoe Trodd and John Stauffer have two, Meteor of War, which includes Brown's writings and the writings of his relatives and contacts, and The Tribunal, which is a compilation mainly of reactions to the raid on Harpers Ferry. I also have a penchant for Du Bois' bio on Brown for its unique perspective. All that being said, Brown was, technically, a domestic terrorist, and yet it's important to take "terrorism" out of a post 9-11 mindset when talking about Brown. We tend to see terrorism as unjustified these days, as an unfair act of violence, as provoked by either madness or blind dogma. Brown has been wrongly accused of both madness and blind dogma, when in fact, although he was an intensely religious man, he was driven by rational and humanitarian goals to defeat a force that was in itself an act of war upon slaves. The horrors of slavery justify Brown's actions, or at least his intentions. Reading Du Bois' description of slavery in chapter 5 of John Brown will help put the picture into perspective. Sure Brown was a terrorist, but that wasn't necessarily a bad thing. Brown was an aging man who saw that his government continued to aid and abet the greatest crime in American history, and he decided to take matters into his own hands. His proposed method was, in fact, far less destructive than a civil war. In his own words - "I, John Brown, am now quite certain that the crimes of this guilty land can never be purged away but with blood. I had as I now think, vainly flattered myself that without very much bloodshed, it might be done."