Reply #11: Here is the official US Army definition

[hack89]

http://www.hks.harvard.edu/cchrp/Use%20of%20Force/October%202002/Parks_final.pdf

Here is the summary.

Summary. Assassination constitutes an act of murder that is prohibited by international law and Executive Order 12333. The purpose of Executive Order 1233 and its predecessors was
to preclude unilateral actions by individual agents or agencies against selected foreign public
officials, and to establish beyond any doubt that the United States does not condone assassination
as an instrument of national policy. Its intent was not to limit lawful self defense options against
legitimate threats to the national security of the United States or individual U.S. citizens. Acting
consistent with the Charter of the United Nations, a decision by the President to employ
clandestine, low visibility or overt military force would not constitute assassination if U.S.
military forces were employed against the combatant forces of another nation, a guerrilla force,
or a terrorist or other organization whose actions pose a threat to the security of the United
States.