Fungibility
Etymology
From the Latin legal phrase res fungibilis, from fungi, "to perform or discharge," as in something that can perform the function of another. It entered English through law, describing goods that can settle a debt by equivalent rather than by the exact item lent. The term surged into wider use with cryptocurrency and especially "non-fungible tokens," which market a claim of digital uniqueness against money’s ordinary interchangeability.
noun · /fun-jih-BIL-ih-tee/
The property of a good whose individual units are perfectly interchangeable, so any one is as good as any other.
What it names
Why a dollar is a dollar but your wedding ring is not just any ring — a distinction that clarifies what can be treated as a commodity and what carries identity that can’t be swapped away.
Example
Money is fungible, so earmarking the bonus "for the vacation fund" is a mental trick — it frees up other cash to spend on anything.
Related idea
Fungibility is about interchangeability of units; liquidity is about how quickly you can convert something to cash without losing value, and a rare painting can be neither fungible nor liquid.