Terrorism in Code Geass is not that much different compared to the real world, although it is interpreted differently.
Terrorism in Code GeassEdit
In the Code Geass universe, terrorism is usually interpreted as Resistance to Britannian Occupation.
Zero is the first main character in canon to be branded as a terrorist after it is revealed that he murdered his brother the Third Prince Clovis, Viceroy of Area 11 at the time of his death. The Black Knights are branded terrorists by the press at first because they used tactics like bombing, gunning down sellers of Refrain, and attacking the Britannian Military.
Towards the end of the story, the Black Knights as an organization are really no longer considered to be terrorists, rather they are called freedom fighters, fighting against the "evil" emperor Lelouch, the man that was once their leader as Zero.
For more information see: Terrorism