I was reading a story on SOL, I think yesterday, and now can't find it, and in the story some character advises that Cantonese is widely spoken (Mandarin being the most used) and then said something like - just don't call Cantonese a dialect.
I realize it's fiction and a character can say anything that is true or untrue - but IS Cantonese a dialect? If not, what is it?
It's a language—unless you want a visa to enter PRC, in which case always publicly call it a dialect (at least until you leave the country).
By the rule-of-thumb test of mutual intelligibility, however, it's a separate language. As are most of the regional speeches of southern China (Min, Hakka, and so on).
Note that from a linguistics point-of-view Cantonese and Mandarin are both considered to be dialects of Chinese.
Chinese is the parent written language which is readable by both child languages, although Cantonese uses the traditional form while Mandarin uses a simplified form. The grammar is the same though, so other than some vocabulary differences both writing systems are readable by speakers of both languages.
The spoken versions of the language are not understandable by speakers of the other language, though.
Note also that it's not just Cantonese and Mandarin. There are at least seven major language groups (depending on definition) that are all child languages of Chinese and most have multiple sub-languages.
For example, the Cantonese is part of the Yue group and is the language spoken in Guangzhou, Wuzhou, Hong Kong, and Macau. The Macau dialect of Cantonese is considered the standard (known in linguistics as the prestige dialect). Depending on the classification system used there are at least five languages in Yue overall and could be a couple of dozen.