One of an interracial group of civil rights activists who rode buses through parts of the South in order to protest racial segregation WordNet 3.0 & Open English WordNet

Freedom Riders were civil rights activists who rode interstate buses into the segregated Southern United States in 1961 and subsequent years to challenge the non-enforcement of the United States Supreme Court decisions Morgan v. Wikipedia

In the United States during 1961, a civil rights activist who participated in one or more trips by bus or other forms of transport, known as freedom rides, through parts of the southern U.S. to demonstrate opposition to racial prejudice and segregation. Wiktionary