This is the big issue with 'defining' pizza and many other Italian dishes - Italians themselves can't decide how it should be and love to debate these things over and over (usually while eating food)!

Carbonara is another highly debated dish - here in Roma (where it is was supposedly born ~50 years ago) some people insist you only need egg, where as others insist you need egg and cheese (and then there is the debate over what cheese). I've heard some restaurants stopped serving the dish to prevent heated arguments with their customers.

Even the word for bags in a supermarket can't be agreed on. Here we use "buste", where as in the north they use "sacchetto". Our word there means envelope, so obviously you will get a funny look if you ask for three envelopes when paying for your groceries (the joke is on us though, we use that word for envelope too :P).

Further reading:

http://www.seriouseats.com/2016/02/origins-history-of-roman-...