How you define terrorism depends on your point of view
If so called terrorists are on your side, or are acting for a cause you believe in you might not call them terrorists at all. You would more likely call them ‘freedom fighters’ or ‘protectors’.
If the terrorist acts are directed against you then you may see them as ‘traitors’ or ‘murderers’.
In Roman times a Jewish group called the Zealots used terrorist tactics against the Romans.
A Muslim group called the Assassins first operated in the 600’s - yes it inspired Assassin's Creed...
The Serbian Black Hand terrorists carried out the assassination which triggered the First World War in 1914. (Gavrilo Princep who killed Archduke Ferdinand)
History has shown that terrorist tactics are a very effective way to attack a larger and stronger enemy. Here the USS Cole is under tow after being holed by a terrorist act.
Terrorist groups have fought against governments, monarchs and foreign enemies. In 1984 the Grand Hotel in Brighton was bombed by the IRA who hoped to kill the Prime Minister and her cabinet who were there for the Conservative Party Conference.
With terrorism forming a significant feature of modern history then we can conclude that terrorist groups find that they consider themselves highly effective in their aim of bringing about political change and putting pressure on governments.
The fact the governments are expending so much effort to fight terrorism shows how worried they are about terrorism and shows they still see terrorists as a potent force.