A demonstration is the public display of the common opinion of a activist group, often economically, political, or socially, by gathering in a crowd, usually at a symbolic place or date, associated with that opinion. The purpose of a demonstration is to show that a significant amount of people are for or against a certain issue, person, law, etc.
activist in a demonstration, October 2004
A demonstration is usually considered more successful the more people participate. A growing trend in the United States has been the implementation of "free speech zones," a fenced-in area which is often far-removed from the event which is being protested; critics of free-speech zones argue that they are unconstitutional by their very nature and that they lessen the impact the demonstration might have otherwise had.
Some demonstrations and riots turn, at least partially, into violence against things (like cars and shops), bystanders and the police. These acts of destruction against private property -- which are not thought to be acts of "violence" by some, since they do not hurt people -- are targeted toward major corporations and chain stores, and rarely affect independently-owned businesses. Police often resort to non-lethal weapons, such as Tasers, rubber-bullets and pepper spray to control the crowd; it is believed by some that they use agent provocateurs to rile the crowd, thereby justifying the use of violence against demonstrators.
See also: Protest, freedom of assembly
A military demonstration is the show of armed forces and their capabilities, often in maneuvers, to impress a potential enemy. It can be used either to provoke that enemy into opening an armed conflict, or to scare him away from one.
A demonstration is a way of teaching, for example, by performing a certain action, so others can see and learn it.
A demonstration is a conclusive proof.
For music or software demonstrations, see demo.