Recently, in a review a debate stated whether we should use
postalCode. Initially, I was drawn torwards the former, however, somebody then pointed out that
zipCode is rather country / domain specific, while
postalCode is the generic term.
A little bit of research indeed showed me that the term "zip code" is not only of US origin, but the identifier that was introduced (and is mainly used) by the United States Postal Service. So it could be considered a special type of a "postal code". That leaves us with taking postal code as the real thing.
So what is zip code then? I think of it as a kind-of genericized trademark, such as Kleenex (or "Tempo" in German). The real term, however, is facial tissue (or "Taschentuch" in German).
Long story short - if you name things keep the attention to detail up, such that generic (non misleading) terms are preferred over specialized (but misleading) terms if we refer to the generic use.