Most commonly accepted definition for "terrorism" is "the unlawful use of force or violence against persons or property to intimidate or coerce a government, civilian population, or any segment thereof, in order to achieve specific political, social, or economic objectives".
Note that the only thing causing the above to be characterized as an act of "terror" is the word "unlawful". If the very same act of aggression is given a resemblance of being justified on legal grounds then it is magically transformed to an "operation" and given a code name (i.e. Panama attack was called Operation Just Cause and the current Iraq invasion began as Operation Iraqi Freedom). In fact there is a website dedicated to generating such code names . Just choose the area and type of action from the two drop down menus and you'll instantly have a nice label to put on your operation. An air strike on Iran, for example, returned a very catchy name of "Intense Sword" and the same act on Russia was sanitized as "Silent Angel".
So how does a country like the U.S. go about taking care of that little "unlawful" problem? Simple; first task a few bureaucrats or political pundits who've not served a day in uniform with writing a paper to be used as blueprint for global domination through pre-emption and military intervention and call it the "The National Security Strategy of the United States." Then label all those persons, organizations, or even states not readily submitting to your will as "terrorists" and strong-arm the international community, the U.N. and the Congress into supporting that strategy in violation of both U.S. and international laws.
Article 6 of the U.S. Constitution establishes that ratified treaties, such as the U.N. Charter, are the "supreme law of the land."
Article 1 of the U.N. Charter  states that "The purposes of the United Nations are to maintain international peace and sovereignty, and to that end: to take effective collective measures for the prevention and removals of threats to peace, and for the suppression of acts of aggression or other breaches of the peace and to bring about by peaceful means, and in conformity with the principles of justice and international law, adjustment or settlement of international disputes or situations which might lead to a breach of the peace. . . "
Article 2 states that all member states "shall act in accordance with the following Principles: ...All members shall settle their international disputes by peaceful means in such a manner that international peace and security, and justice, are not endangered". "All members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations...."
One can see that under such a framework, all acts of aggression are to be suppressed and force can only be used as a last and unavoidable resort. The U.N. Charter was enacted in 1945 in the aftermath of the devastation from World War II in order to bring an end to acts of aggression and "to save succeeding generations from the scourge of war, which twice in our lifetime has brought untold sorrow to mankind."
Branding of Iran's Revolutionary Guard Corps as a terrorist organization is a blatantly illegal and deliberate act to prepare the ground for waging a pre-emptive war of aggression against yet another sovereign government and its civilian population in order to achieve specific political, social, or economic objectives.