Fungible and Non-fungible Security

The terms "fungible" and "non-fungible" relate to the way in which securities are held.
If an asset is fungible, this means that when such an asset is lent, or placed with a custodian, it is customary for the borrower or custodian to be obliged at the end of the loan or custody arrangement to return assets equivalent to the original asset, rather than the identical asset. In other words, the redelivery of fungibles is equivalent and not in specie (identical).
Undivided securities are always fungible by logical necessity. Divided securities may or may not be fungible, depending on market practice. The clear trend is towards fungible arrangements.