I think it maps decently well to the concept of a currency. It has no intrinsic value, and the amount of it in circulation is controlled by a central authority.
In the case of the US, the Fed constantly adjusts the dollars in circulation to further their monetary policies (typically low inflation, with economic growth), while in the case of BitCoin, the amount in circulation is set by a mathematical curve that is deflationary in nature.
Analyzing it as a deflationary currency (which many people treat as an "investment", because it increases in value over time) is about right.
Currencies have additional characteristics in the real world, that are not part of the dictionary definition:
- I can file a police report if my dollars are stolen, and there is a non-trivial chance I could get them back.
- I can store dollars at an insured institution, with a high degree of confidence in retrieving them again. [1]
- I [get to | have to] pay my taxes in dollars.
In essence, for all of our lifetimes, "currency" has always meant "state currency", with all the good and bad that goes with it. Even when precious metals were the norm, states still tended to get involved (stamping the Emperor's face on certain coins and giving those coins preferential treatment).
While BTC meets the definition for currency in some ways, it's probably more practical in our current financial ecosystem to think of them as a highly fungible (and volatile) asset. (As with the stock market, no one should be putting all their life savings into crypto-coins.) As the ecosystem continues to grow, this may change.
[1] This might be the balancing factor to the deflationary issue, to the extent that it matters: that most people might end up keeping their hoard at an insured, well-secured BTC Bank, and paying a small steady percentage for the assurance that their whole pile doesn't disappear.
In the case of the US, the Fed constantly adjusts the dollars in circulation to further their monetary policies (typically low inflation, with economic growth), while in the case of BitCoin, the amount in circulation is set by a mathematical curve that is deflationary in nature.
Analyzing it as a deflationary currency (which many people treat as an "investment", because it increases in value over time) is about right.