The answers to these questions are simple: Yes, bitcoin is a currency, but we cannot know if it will remain so in the future. It does, however, have many properties that might make it viable in the long run.

There should be no controversy anymore about calling bitcoin a currency. A currency is simply a good that serves as a medium of exchange, meaning that people trade it for the goods and services they want. In different circumstances, different goods serve this purpose. In prison, cigarettes are used as currency. In the early American colonies, tobacco was a currency. Bitcoins are used online as a medium of exchange for thousands of people around the globe.

Now, whether or not it is a "sound" currency is an entirely different question. In order to make that judgment, we need to understand the properties that make currencies sound or unsound in the first place. Consider gold and silver: They have persisted as a medium of exchange throughout history, and not by chance. They have desirable properties.

A sound currency needs to stay valuable over time, have a limited supply, and be easily divisible and portable. If any one of these properties is missing, it is reasonable to question the long-term viability of such a currency, especially if it is subject to competition. This is why many free-market economists are called "goldbugs"—they see the shortcomings of fiat money in comparison to hard-money alternatives.