CSV stands for comma separated values, so the comma is an official CSV delimiter. But sometimes, the semicolon is used as a CSV separator.
Comma is usually used as a CSV separator worldwide, in US, UK, North American countries etc. But from my experience, some CSV files received from these countries were semicolon separated. So using of comma as a CSV delimiter, is not a "rule" in these countries.
Semicolon is often used as a CSV separator in some European countries, because comma is used for decimal symbol.