What is a Eurodollar?

The Eurodollar is a US dollar created outside of the US-controlled banking system. For example, a bank in Europe creating a loan denominated in US dollars. Eurodollars are US Dollars and have nothing to do with Euros.  

These dollars are not reflected in the FED data. The significance to Bitcoin is it is another aspect of the US dollar-based fiat system that has potentially unchecked currency inflation (increase of the amount of dollars in existence). This inflation lowers the value for everyone else in the system. There are other aspects of Eurodollars beyond the scope of this answer.