Progressive, as a descriptor in American politics, has evolved over time. During the Progressive Era in the early 20th century, it was a synonym for modernism.
From the 1940s through the Cold War, it tended to be euphemistic for far-left groups and ideology. While all progressive groups of the time certainly were not affiliated with Communism, some domestic groups identifying as progressive were Communist fronts. During the Korean War and Vietnam War, the North Koreans and North Vietnamese called those prisoners of war who cooperated "progressive".
In current usage, the term is used more as a more centrist, but still left-oriented, balance to conservative activism, especially neoconservatives and the Christian Right. The Congressional Progressive Caucus is the largest sub-caucus within the Democratic majority in the U.S. House of Representatives.