From Traderpedia

Definition: Originally referred to US dollar-denominated deposits in commercial banks located in Europe. Over time, the term came to include deposits in a commercial bank in any country denominated in any currency other than that of the country.

Eurodollar trading represents the perceived value of these dollar deposits, which differ from direct foreign exchange trading. Most of the Eurodollar deposits are in London banks but it is possible to have Eurodollar deposits anywhere other than the US For example, a Euroyen deposit represents the commercial deposits in Yen outside Japan.