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This term refers to a soundtrack replaced or voice track added to an existing film, to make it understandable in a different language. The most common usage is for Japanese anime to be dubbed into English, but the term is also used for any media made available in a different language from the original. It's an alternative to subbing.
The term Dub is a general one, referring to both dubs created by the original creators, and dubs created by fans. The term Fandub specifically refers to those created by fans, usually without authorization from the copyright holders of the original source.