PostgreSQL equivalent of the Oracale 'unique' qualifier

I hope the subject says it all.  I'm porting an Oracle-centric
application to PostgreSQL and the Oracle sql is full of the 'unique'
qualifier.  I'm assuming PostgreSQL does not support 'unique' since
don't see a 'unique' anywhere in the PostgreSQL docs.  Is there a
substitute or a technique to get the same result?

Thanks,
-=bill stafford