The Abbey in Oakwood - Caspar David Friedrich 

Since 2011, this blog has mainly been using Dr Boaz Ganor's Definition of Terrorism:

"Terrorism is the intentional use of, or threat to use, violence against civilians or against civilian targets, in order to attain political aims (..) based on three important elements:

The essence of the activity (i.e. an activity that doesn't involve the use, or threat to use, violence will not be defined as terrorism).
The aim of the activity is always political (i.e. in the absence of a political goal, the activity in quest will not be defined as terrorism). 
The targets are civilians (i.e. acts are purposely directed at civilians; which distinguishes terrorist acts from other kinds of political violence [e.g. guerrilla warfare and civil insurrection]."