No. The dictionary defines this very clearly. "Murder" is the killing of a human under conditions specifically set forth in law. In the USA that includes deliberate intent or premeditation, or occurring during the commission of a serious crime. (U.S. law distinguishes between first-degree and second-degree murder but there's no need to go into that here.) Again, no. A vigilante is one who takes the law into his own hands, for example for exacting revenge without bothering to notify the authorities, waiting for a trial, and hoping the defendant is found guilty. Once again, no. Execution is the carrying out of a judgment or a sentence directed by a formal court. In the USA it must be handed down by a judge. Hey, I despise cops as much as the next unrepentant hippie, but once again this is the Linguistics subforum so we are obliged to play by the rules of the language. In the USA, the police are an organized civil force (i.e., recruited, vetted, selected and directed by the civilian branch of government) for maintaining order, preventing and detecting crime, and enforcing the laws. You're batting zero today. An army is the military force of a nation (i.e., recruited, vetted, selected and directed by the military branch of the government), trained and armed for war, and ready to make war (in the USA, at least theoretically) when ordered by the civilian government . "Terrorism" is a relatively new word with no stable consensus definition. However, one that most would accept is "the use of violence and threats to intimidate or coerce, especially for political purposes." A terrorist is, therefore, one who uses violence and threats to intimidate or coerce those he regards as his enemies, especially for political purposes. You're finally more-or-less right. More clearly stated: an underground organization composed of groups of private individuals, working as an opposition force in a conquered country to overthrow the occupying power, usually by acts of sabotage, guerrilla warfare, etc. The American definition focuses on "a battle against established forces of tyranny and dictatorship." The British definition is considerably different in spirit, not just wording: "a militant revolutionary." This definition includes rebels who attempt to bring down any established government, even one that the vast majority of their countrymen regard as democratic and benevolent. Many of today's "freedom fighters" would insist that they are fighting the battle described in the American definition, but their countrymen would insist that they are "militant revolutionaries" who do not represent the will of the populace, but instead are hoping to install a government which the populace regards as abhorrent. This surely applies to most of today's Islamic terrorist groups, as well as the militias of the drug lords in Latin America.