melting pot

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A container in which a substance is melted.
• noun A place or society in which immigrants of different cultures or races form a single culture.

from The Century Dictionary.

• noun A crucible.

from the GNU version of the Collaborative International Dictionary of English.

• noun A vessel in which anything is melted; a crucible.
• noun (Sociology) A place where people of different backgrounds become similar in culture. The United States has often been referred to as a melting pot, though the differences in cultures of recently arrived immigrants persists beyond the generation of immigrants.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A crucible, or similar pot, used to fuse mixtures of metals etc.
• noun idiomatic A place where many divergent things (often races or cultures, but also talents) come together and are homogenized.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a vessel made of material that does not melt easily; used for high temperature chemical reactions
• noun an environment in which many ideas and races are socially assimilated