What is the definition of terrorism? Who has the right to define it?

I always hear about terrorism and terrorists and it often goes (at least in the media) with a Muslim or someone with an Arabic name. My definition of terrorism is: Killing of innocent people. Simple as that. Based on this definition, there are and have been a lot of terrorists in the world. Bin Laden, Bush, Blair, Netanyahu, El-Bashir, Al Assad, King Fahad of Saudi Arabia, Timothy McVeigh, Anders Breivig just to name a few. How come the people of the media use the term terrorist for Al Assad and not for Bush?