Hard Money Definition:

Hard money is any form of money that is likely to hold its value over time through its inherent difficulty to increase in supply. - Stormrake 2022

Explanation: 

Money has existed in many forms throughout human history. Anything can take the form of money as long as it can be traded. Throughout history we have seen that no matter what authority issues money or how strong the link between money and trade, the easier the money is to inflate - the faster it loses value over time. Examples are numerous but our favourite might be seashells. As crazy as it may seem today seashells were used as money all over the world from Africa & Europe to Australia. Shells were popular as they were easily transportable and commonly available. However, pretty soon it became obvious that you'd be better off going to the beach and looking for shells than working in your 9-5 (or whatever hours people kept back then). As soon as supply increases due to it being too easy to collect shells - the value of shells falls against all forms of better money (including scarce assets). Just like shells, other forms of soft money continuously fall against the value of hard money which is hard to inflate.
 

Gresham's Law definition

Gresham’s law, observation in economics that “bad money drives out good.” More exactly, if coins containing metal of different value have the same value as legal tender, the coins composed of the cheaper metal will be used for payment, while those made of more expensive metal will be hoarded or exported and thus tend to disappear from circulation.
- Britannica

Explanation:

People hold on to scarce hard money and spend easy money. People in central and southern America know this rule too well. For Venezuelans, it is imperative to spend their Bolivars asap (before the prices rise further) and to keep their US dollars. They know that dollars are more likely to hold value over time because its supply is not being inflated as badly as the Bolivar. 

The Hardest Money of all:

Bitcoin has the following characteristics making it the hardest money ever.

Total Maximum Supply:

21 Million bitcoins - that's it. No other money can say that there is an exact finite supply. On top of that all lost bitcoins reduce the maximum supply so the real achievable supply is 10-20% lower.

Reducing Inflation:

Over 90% of the 21 Million has already been released meaning there is less than 10% to release over the next 100+ years. Every 4 years the supply per block halves and the money becomes even harder. The next halving event is less than 2 years away.

Decentralisation of Control:

The entire supply mechanism is secured by unbreakable encryption and decentralised, meaning no one has control. That's a huge feature because it means you don't need to trust anyone to not debase your money. No one has the power to issue more bitcoins than is already programmed.