Does the United Nations expressively define an "act of aggression"?

Answer 1

Who cares?

Answer 2

The UN Charter provides for the definition of the act of aggression but it does not apply to the US as the main financier.

Answer 3

The United Nations has define what the term "act of aggression" means in the UN Charter, GA Resolution 3314 in 1974, and the Rome Statute that created the ICC. Their definitions are listed below:

The United Nations Charter
The relevant provisions of the Charter of the United Nations mentioned in the RSICC article 5.2 were framed to include the Nuremberg Principles. The specific principle is Principle VI.a "Crimes against peace", which was based on the provisions of the London Charter of the International Military Tribunal that was issued in 1945 and formed the basis for the post World War II war crime trials. The Charters provisions based on the Nuremberg Principle VI.a are:

Article 1:
The Purposes of the United Nations are:
To maintain international peace and security, and to that end: to take effective collective measures for the prevention and removal of threats to the 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;
To develop friendly relations among nations based on respect for the principle of equal rights and self-determination of peoples, and to take other appropriate measures to strengthen universal peace;
Article 2, paragraph 4
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.
Article 33
The parties to any dispute, the continuance of which is likely to endanger the maintenance of international peace and security, shall, first of all, seek a solution by negotiation, enquiry, mediation, conciliation, arbitration, judicial settlement, resort to regional agencies or arrangements, or other peaceful means of their own choice.
The Security Council shall, when it deems necessary, call upon the parties to settle their dispute by such means.
Article 39
The Security Council shall determine the existence of any threat to the peace, breach of the peace, or act of aggression and shall make recommendations, or decide what measures shall be taken in accordance with Articles 41 and 42, to maintain or restore international peace and security.

General Assembly Resolution 3314
On December 14, 1974, the United Nations General Assembly adopted Resolution 3314, which defined the crime of aggression. This definition is not binding under international law, but it is often cited in opposition to military actions.

This definition makes a distinction between aggression (which "gives rise to international responsibility") and war of aggression (which is "a crime against international peace"). Acts of aggression are defined as armed invasions or attacks, bombardments, blockades, armed violations of territory, permitting other states to use one's own territory to perpetrate acts of aggression and the employment of armed irregulars or mercenaries to carry out acts of aggression. A war of aggression is a series of acts committed with a sustained intent. The definition's distinction between an act of aggression and a war of aggression make it clear that not every act of aggression would constitute a crime against peace; only war of aggression does. States would nonetheless be held responsible for acts of aggression.

Rome Statute of the International Criminal Court
The Rome Statute of the International Criminal Court lists the crime of aggression as one of the most serious crimes of concern to the international community, and provides that the crime falls within the jurisdiction of the International Criminal Court (ICC).

However, Article 5.2 of the Rome Statute states that "The Court shall exercise jurisdiction over the crime of aggression once a provision is adopted in accordance with articles 121 and 123 defining the crime and setting out the conditions under which the Court shall exercise jurisdiction with respect to this crime. Such a provision shall be consistent with the relevant provisions of the Charter of the United Nations." The Assembly of States Parties of the ICC may adopt such a definition at a review conference scheduled for 2009.