The Use of indiscriminate violence by private (non-state) groups to cause mass fear and panic with the ostencible purpose of advancing revolutionary political goals, but often expressing a more prosaic criminal element and motivation as well.  Through seldom without purpose or internal reationality, it is nonetheless alway outside the norms of civilized society.  In the 19th century anarchists (especially Russians and Spaniards) were the dominant group engaged in terrorism. In the 1920's and 1930's most terrorists came from the extreme right-wing.  Thus, the League of Nations drafted the first international convention on terrorism in 1937, but it never came into force. There actions were in responce to the assassination of Alexander I of Yugoslavia by a Macedonian Nationalist, who fled to, and received support from. fascist Italy.

In the 1960's and 1970's most active terrorist were on the left, or had agendas dirived from the several conflicts in the Middle East.  The United Nations General Assembly (UNGA) and several regional organizations responded with conventions making terrorism a crime. The main The United Natins (Outside Link) instrument came into force in 1977.  Other conventions followed in the 1980s and 1990's.

The problem is that it is very hard for any international body to come up with laws against Terrorism that will satisy all of the members of the orgainzation and also still allow terrorism to flourish where it is a necessary or the only tool of change.