Currency is a unit.
It is a unit in the system of measurement of value. Just like an inch is a unit in the system of measurement for length.
Units (and there have been thousands of different ones) all have 2 things in common.
1- They are not real. They are all made up, everyone of them is just an opinion. Inch, pound, meter, cat 5, g force,currency --- not real.
the are all just an opinion that we chose to share. We all share the opinion that an inch is so long, if we all shared the opinion that an inch was a foot then it would be.
2- All units are equal to a constant.
Bitcoin is not a currency because it is not a unit. It has to be definable it has to be equal to something. An inch is not real but it is definable.
Hello!
I think you might find it interesting to look into the history of measurements. We have had all kinds of units over the millennia, and our system of units is constantly evolving. Even the SI units which are now used as the common standard in scientific measurements are subject to change.
So you may find that some of the things we currently rely on as 'units' don't actually fit your definition of 'unit'.
Value is probably one of the more complicated things to assign a fixed unit to, because it is so subjective. But I think you're wrong about Bitcoin not being a unit. We have a fixed supply of 21 million Bitcoins. 1 Bitcoin remains constant (i.e. 1 BTC will always = 1 BTC) - its up to us to decide how much we value it.