Shared hallucination.

(To those that might dismiss this as snark, look up the definition of currency. And if you find that adventure interesting follow up by reading the excellent book “Sapiens: A Brief History of Humankind” by Yuval Noah Harari for an easy read that thoroughly explains the concept in an afternoon of reading. In that book it is called “shared delusions” instead.)

Currency by itself, yes. Gold is probably best example.

Most fiat currencies impose a degree of solidity by being required for payment for certain services (read: taxes)

> In that book it is called “shared delusions” instead.

Or, by less casual name, "intersubjectivity". And yes, it's a very important idea that directly applies here.

Perhaps even a must for any human society to exist at all...

Legal system.