Bitcoin can be a currency if it is stable and trusted among most of people and has a value that slightly changes with time
"A currency, in the most specific sense is money in any form when in use or circulation as a medium of exchange, especially circulating banknotes and coins." -Wikipedia
I can confirm that Bitcoin is already a currency as it is used to trade for different goods and services. For example on exchanges or like on Bitcointalk for services.
Thousands of online services accept Bitcoin as a payment method. Just due to people mindset that would prefer old school payment methods, but future is around us. Let's look into it.