1. In science, a melting pot is a container where substances are melted.
2. In the social sciences, assimilation is the process of integration, whereby, immigrants (and other minority groups) are "absorbed" into an integrated mainstream society. A region, where assimilation is occurring, might be known as a "melting pot". Such a "melting pot" can result in a relatively homogenous society, with a strong sense of nationalism; however, where minorities are strongly urged to assimilate, there can arise groups which directly oppose integration.
Perhaps the best known application of this term is in regards to the United States. In 1879, Army Captain Richard Pratt opened the United States Indian Training and Industrial School in Carlisle, Pennsylvania. The school integrated Indians into white society, and took "before and after" photographs. A competing approach is multiculturalism. This approach is to some extent used in Canada.
The metaphor of the melting pot comes from Englishman Israel Zangwill's play The Melting Pot, which was first performed in Washington, D.C in 1908.