Original: The term specifically describes activities associated with the period in the United States known as the Second Red Scare, lasting roughly from the late 1940s to the late 1950s and characterized by heightened fears of communist influence on American institutions and espionage by Soviet agents.
Simplified: McCarthyism is the term describing a period of intense anti-Communist suspicion in the United States that lasted roughly from the late 1940s to the mid to late 1950s.