The term "melting pot" is a metaphor for a heterogeneous society becoming more homogeneous, the different elements "melting together" into a harmonious whole with a common culture.
The exact term "melting pot" came into general usage in the United States after the 1908 play The Melting Pot by the Jewish Israel Zangwill. However, Israel Zangwill was also an important proponent of creating a separate Jewish homeland. This may seem contrary to the desirability of a "melting pot" society (at least for Jews).
American industrialist Henry Ford promoted the "melting pot" idea among his immigrant work force. Ford set-up and English School for his foreign-born workers. After their graduation from the school the students participated in a "Melting Pot Ceremony" where the immigrant would enter into a large "melting pot" waving his native country’s flag and emerge waving an American flag.
That a "melting pot" is possible for all groups is dubious. See magic dirt.
The "melting pot" model has been criticized by proponents of multiculturalism. They have suggested alternative metaphors to describe the current American society, such as a "mosaic", "salad bowl", or "kaleidoscope".