Unique—Wolfram Language Documentation [proxy]

generates a new symbol, whose name is of the form $nnn.

generates a new symbol, with a name of the form x$nnn.

Unique[{x,y,…}]

generates a list of new symbols.

Unique["xxx"]

generates a new symbol, with a name of the form xxxnnn.

Details

Unique[x] numbers the symbols it creates using $ModuleNumber, and increments $ModuleNumber every time it is called.
Unique["xxx"] numbers the symbols it creates sequentially, starting at 1 for each string xxx.
Unique[name,{attr₁,attr₂,…}] generates a symbol that has the attributes attr_i.

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Create a unique symbol with no particular name:

Create a unique symbol whose name begins with x:

Create a unique symbol in the way Module does: