Both these words have the same meaning. However "rabat" is more formal. Other alternatives are: opust and upust (both are less common)

Actually, the meaning is the same. We can use both forms but in some situations, we use one form more often than the other. It depends on the fact which one is in custom.

Synonyms: rabat, bonifikata; obniżka; opust; promocja; przecena; ulga; upust; zniżka.