Terrorists have been traditionally defined as vigilantes who use threats or violence against civilians in order to attain political, religious and/or ideological goals, by process of fear, intimidation, and/or coercion. These days you can be exempt from the label according to ethnicity, as the satirical graph on the right shows.
This wasn’t true before 9/11. Ted Kaczynski (“the Unabomber”) opposed modern technology, and mailed bombs to people between 1978 and 1995, and Timothy McVeigh was a white supremacist and New Order conspiracy theorist who blew up the Alfred P. Murrah Building in Oklahoma City in 1995. They were called terrorists by every mainstream media outlet. Today they probably wouldn’t be.
Consider Daniel Cowart (the white supremacist who plotted to kill black school students and then President Obama in 2008), James Von Brunn (the Holocaust Memorial Museum shooter in 2009), Byron Williams (the Tea Party member inspired by the propaganda of Glenn Beck, who armed himself and went “liberal-hunting” in 2010), and Jared Loughner (the conspiracy theorist believing in a governmental plot to brainwash people, who killed six people in 2011). The terrorist activities of these men speak for themselves, despite the reluctance to call them terrorists in a post-9/11 world.
We should acknowledge the reason for that reluctance. Terrorisms have different manifestations and threat levels. To lump the above men together creates the misleading impression that there is a concerted movement equivalent to the Islamic jihad. Jihad is a massive global threat first of all. In the U.S. there have been many foiled jihadist plots, and if we included those in comparing the number of Islamic terrorist attacks vs. non-Islamic, the former would dwarf the latter. And when you factor that whites comprise 77% of the U.S. population, and Muslims less than 1%, that magnifies the jihad threat exponentially.
It doesn’t take fear-mongering to recognize that Islamic terrorists are a persistently dangerous breed, but that doesn’t warrant making them the “only” terrorists. That strips the definition and fuels counter-productive political narratives. Dylann Roof is as much a terrorist as Omar Mateen.