what is the difference between UNIQUE INDEX and UNIQUE KEY?

I have seen couple of sampl开发者_高级运维e tables one is with UNIQUE INDEX and other is with UNIQUE KEY. what is the difference between two?? or both are same?

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CREATE TABLE
KEY is normally a synonym for INDEX.

You can use them interchangeably as described in syntax page
[CONSTRAINT [symbol]] UNIQUE [INDEX|KEY]