Transferable security
A negotiable instrument: A share, bond, note, warrant, certificate of deposit — that kind of thing — evidencing indebtedness or an equity interest in an undertaking, in bearer or registered form and which the holder may freely transfer by delivery. These days transferable securities are cleared electronically through clearing systems like Euroclear, Clearstream and DTC. The days of security-printed bearer bonds are over.
To be contrasted with indebtedness or exposure in the form of, say, a loan or over-the-counter derivative, where creditor/counterparty cannot easily sell its right to repayment. (Yes, yes, I know it can novate it, or sub-participate it).
Also called a physical security (to distinguish it from a synthetic one.