It’s somewhat fashionable to label any number in a game a currency. In practise, however, it gets a little confusing.
Anyone can grasp the idea that in-game gold is a currency, but what about your character’s health or experience points? Advocates might say that the player is trading health for damage or progress during the course of the game. But to most people that’s pretty tenuous.
I prefer to think of currency as one type of resource instead. This is a post about different types of resources that you could use in your game (including currency) and some guidelines on how to use them well.