McCarthyism, named after Joseph McCarthy, was a period of intense anticommunism, also (popularly) known as the (second) Red Scare, which occurred in the United States from 1948 to about 1956 (or later), when the government of the United States was actively engaged in suppression of the Communist Party USA, its leadership, and others suspected of being communists. From the viewpoint of many conservative American citizens at the time, the suppression of radicalism and radical organizations in the United States was a struggle against a dangerous subversive element controlled by a foreign power that posed a real danger to the security of the country, thus justifying extreme, possibly illegal measures. From a radical leftist viewpoint it was probably seen as class warfare, particularly by the actual communists targeted. From the viewpoint of people who were caught up in the conflict for simply being objectionable (but not communist spies), it was a massive violation of civil and Constitutional rights.

McCarthy's anticommunist crusade faltered in 1954 as his hearings were televised, for the first time on the new American Broadcasting Company. ABC needed to fill their afternoon slots, which allowed the public and press to view first-hand McCarthy's badgering of individuals and controversial tactics . The press also started to run stories about how McCarthy ruined many people's lives with accusations that were in some cases insufficiently backed up by evidence.

Since the time of the red scare led by Joseph McCarthy, the term McCarthyism has entered the American vernacular as a general term for the phenomenon of mass pressure, harassment, or blacklisting used to instill conformity with prevailing political beliefs. The act of making insufficiently supported accusations or engaging in unfair investigatory methods against a person as a purported attempt to unfairly silence or discredit them is often referred to as McCarthyism. The Arthur Miller play The Crucible, written during the McCarthy era, used the Salem witch trials as a metaphor for the McCarthyism of the 1950s, suggesting that the process of McCarthyism-style persecution can occur at any time and place. The term "McCarthyism" has since come to mean a government witch hunt seeking to punish unapproved thoughts or political stances.

The term originated in a March 29, 1950 political cartoon by Washington Post editorial cartoonist Herbert Block. The cartoon depicted four leading Republicans trying to push an elephant (the traditional symbol of the Republican party) to stand on a teetering stack of ten tar buckets, the topmost of which was labelled "McCarthyism". The reluctant elephant was quoted in the caption as saying "You mean I'm supposed to stand on that?"