Unique

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

Unique[x] 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. 

Please visit the official Wolfram Language Reference for more details and examples on core symbols.