What is a terrorist?

Sometimes you have to put your political preferences aside and focus on what is going on in America. Killing Americans on American soil because they may be terrorists is scary to me. What would then constitute a terrorist? Think about that question. This truly scares me due to the fact that who knows what the hell equates terrorist. Is it a person who is Muslim? A libertarian? Someone who is anti-governement?  Feel free to peruse the links below.
http://reason.com/blog/2013/03/07/john-mccain-and-lindsey-graham-declare-w

http://www.nationaljournal.com/congress/why-rand-paul-wouldn-t-stop-talking-20130306

I know I am not skilled in this matter, but who really is? Its easier to refrain from your political agendas and view this situation with an open mind. Because honestly, it scares the heck out of me.