Freedom fighter is an often propagandistic term for someone rebelling against an established government that is believed to be oppressive and illegitimate. The term freedom is politically charged and ambiguous (as is oppression); therefore it is often controversial which side in an armed conflict represents the cause of freedom. Though the literal meaning of the words could include anyone who fights for freedom, common use is restricted to those who are actively involved in an armed rebellion rather than those who "fight" for freedom by peaceful means (though such persons often use the term for themselves in a metaphorical sense).
Historically, we find that people who are described as freedom fighters by supporters are called rebels or terrorists by their opponents. During the Cold War, the term freedom fighter was widely used by the United States and other Western Bloc countries to describe rebels in countries controlled by Communist governments or otherwise under the influence of the Soviet Union, including Hungary, Nicaragua, and Afghanistan. The Soviet Union used the term in the same way, to describe rebel movements in countries controlled by or under the influence of the United States and other Western Bloc countries, such as Vietnam, Nicaragua, and Colombia. These rebels often used guerrilla tactics.
The term, while indicating favor of some political group, often does not reflect any actual political position of those fighting. For example, to many people around the world, the leftist Sandinistas were freedom fighters. After their revolution took Nicaragua, the CIA funded a new opposition, the Contras, who were labeled as freedom fighters by the United States government and rebels or terrorists by the Sandinistas and Soviet Union.
The ambiguity of the term freedom makes the use of the label freedom fighter particularly useful for propaganda purposes. It is relatively simple to show that the "enemy" has done something which violates one of the many possible meanings of the word freedom, which allows the propagandist to appear to take the moral high ground by fighting for the cause of freedom. In addition to this, propagandists commonly use virtue words like freedom, which tend to evoke positive images in the target audience in order attach those images and feelings to his cause.