The euro (sign: €) was a currency used by many countries in Europe.
During Day 2, when Marcus, Rouse and Cole broke into Kate Warner's house, she opened the safe for them and Marcus found the wad of €20,000 worth euro banknotes. He asked what they were, and Kate explained that it was European currency, but Marcus threw it aside, asking if she thought he looked European. ("Day 2: 3:00am-4:00am")
Callum Trent had stacks of euros in his safe on board the Barataria. It was the currency used by Karl Rask and Hakeem el-Jamal during their deal over nuclear missiles. (Rogue)