Fungible (adj); capable of being replaced, exchangeable, interchangeable.
From Medieval Latin fungibilis, from Latin fungi “to perform” (e.g., fungor “I perform”) + ible “able to”. In other words, able to be performed, substituting one thing for another.
In finance, money is fungible (e.g., a $20 bill is exchangeable for twenty $1 bills, and both are exchangeable for any product valued at $20 even though the product itself may not be fungible).
Fungible shares a linguistic (and mathematic) history with Function, also from Latin fungi (nominative functio). In other words, to perform or to execute one thing into another. In mathematics, the input set of a pure function could be described as fungible (such as in finance or cryptocurrency).