A collective trauma is a psychological effect on an entire society. Traumatic events that emotionally touch nearly every member of said society will often induce a collective trauma. As a result of such trauma, often media, politicians and, in some extreme instances, the general public, will start looking for scapegoats. Collective traumas often give rise to conspiracy theories.
In hindsight, collective traumas will often be watersheds of history and signify or initiate times of change in the society affected.
In all likelihood, the destruction of the World Trade Center in the September 11, 2001 Terrorist Attack will induce a severe collective trauma in US society, and indeed affect the entire world through primary and secondary reactions.