In forex trading terminology, cross currency refers to a pair of currencies that do not include the U.S. dollar. It is commonplace in the forex market to exchange any foreign currency to U.S. dollars before trading. In cross currency, a trader does not need to go through that.
Cross currency is a technique that aims to completely bypass the need to convert currency to American dollars before converting it back to the desired foreign currency. One example is the GBP/JPY (British pound-Japanese yen) cross for England and Japan currencies. This is invented in order to convert money between the two currencies without needing to convert them into U.S. dollars.