I have never really bought the prevailing definition of terrorism because it seems slippery and self-interested on the part of those who generally use it. It seems to me that people who represent the establishment refer to almost anything that doesn't have state sanction and questions that establishment as 'terrorism.' At the very least people think that the realm of violence is solely legitimate when it is sanctioned by the state and all other politically motivated violence is 'terrorism.' The problem with this definition is, of course, that if a state carpet bombs a country like, say, Vietnam, that is the legitimate violence; but if people in that country defend themselves with any form of violence once their state apparatus has failed suddenly they are terrorists. Thus by the standard definition, George Washington and Nelson Mandela were both terrorists and Arthur 'Bomber' Harris and Brigadier-General Reginald Dyer were both noble soldiers. Anyone who doesn't have a problem with this should reexamine their ethical framework.
(This definition of terrorism is really about power. It is like trying distinguish between legitimate 'religions' and so-called 'cults.' Cults become religions as soon as they have a large enough following and enough money to be called a religion.)
Now, the G20 and the primary countries of that institution have long pursued policies which have knowingly caused terrible suffering, starvation, and environmental disaster. Everything from the so-called EU Common Agriculture Policy which involved food dumping in Africa to Trade Policies which have specifically aimed at maintaining the advantage over less-developed nations have resulted in continued suffering and starvation in the "third world." Meanwhile countries like the US have pursued military efforts in countries like Iraq which have resulted in literally hundreds of thousands of civilian deaths.
Other people define terrorism more simply as the use of violence to gain a political end. I think this is a better, and certainly more functional definition because it does not fall victim to the dubious question of the state having the sole legitimacy in the use of violence.
Meanwhile, democracy is slowly being eroded in our own country and elsewhere in the Western world and our so-called leaders increasingly gain and maintain their power through the undue influence of money and an elite controlled media. And the recent events in Toronto demonstrate that they seek to further legitimize and maintain their power through indiscriminate violence perpetrated by para-military police units. These para-military units knowingly and willfully attacked peaceful, middle-class folks who did nothing but sit on the grass and calmly voice their concern over the kinds of policies that the G20 pursue. It takes little imagination to understand why they did this; to foster fear and terror among the general population and squash political dissent. Many people define terrorism as the indiscriminate use of violence to spread fear in the population to for a particular political purpose. By this definition the police units operating at Toronto were simply terrorists. Their goal was to intimidate people from questioning the legitimacy and morality of the G20, the policies they pursue, and the elites who call the shots.
I have said it before and it needs reiterating; if democracy survives and flourishes it is those that have actively opposed the G20 who will be the historical heroes and the police who will be cast in the role of historical villains much like the Swiss Grenadiers who guarded the Bastille from the Paris Peasants.