A Definition of "Terrorism"

What does it mean? Why is one man's terrorist another man's freedom fighter? Why is it called "terrorism" when insurgents in Iraq kill innocent people in the streets, but not when U.S warplanes bomb a wedding party in Afghanistan? Why was the Washington D.C. sniper called a terrorist, but the kids at Columbine weren't?

This is a serious question. I'm not using this post to criticize our military by calling them terrorists, as some have done. I just want to make sure that we know what we mean when we call someone a terrorist.

Because unless we have a firm definition of terrorism, we will always be swayed by the whims of politicians and the media, who seem to be able to attach that label to anyone they want us to hate.

Help me out. What are your ideas?