According to Hasbro's official Scrabble dictionary, the definition of za is that it is a term for pizza. You can also use the plural, wham. The Merriam-Webster dictionary agrees and points out that it is a slang term. The online Oxford Living Dictionary places an apostrophe in front, “za” to demonstrate that it is an informal and abbreviated term for pizza.
The origin of za dates back to around 1968, when it was used by students in the United States.