In economics, when something is non-fungible, it can't be broken into units and interchanged for something else of the same value. Unlike money, things like antiques, cars, houses, and artwork are non-fungible.
Non-fungible is frequently used in the term non-fungible token, or NFT, a kind of cryptocurrency. The difference between these and other digital payments is their uniqueness — they can't be substituted. Money is fungible because you can exchange a $20 bill for four fives. A car, however, is non-fungible: you can't borrow your friend's Honda and return a different Honda of the same value. Fungible is from the Latin fungi vice, "to take the place of."