The 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 or vice versa, for a homogeneous society becoming more heterogeneous through the influx of foreign elements with different cultural background with a potential creation of disharmony with the previous culture. Historically, it is often used to describe the assimilation of immigrants to the United States.  The melting-together metaphor was in use by the 1780s.   The exact term "melting pot" came into general usage in the United States after it was used as a metaphor describing a fusion of nationalities, cultures and ethnicities in the 1908 play of the same name.
The desirability of assimilation and the melting pot model has been reconsidered by proponents of multiculturalism ,   who have suggested alternative metaphors to describe the current American society, such as a mosaic , salad bowl , or kaleidoscope , in which different cultures mix, but remain distinct in some aspects.    Others argue that cultural assimilation is important to the maintenance of national unity, and should be promoted.