A popular front is any coalition of working-class and/or middle-class entities, including liberal and social democratic ones, united for a purpose. Generally, it is "a coalition especially of leftist political parties against a common opponent". The phrase uses "front" in the sense of a political movement "linking divergent elements to achieve common objectives".
The term was first used in the mid-1930s in Europe by communists concerned over the rapid growth of fascist movements in Italy and Germany, which they sought to combat by coalescing with non-communist political groupings they had previously attacked as enemies.