The ability to be sold or transferred to another party as a form of payment....
Capable of being negotiated; transferable by assignment or indorsement to another person; as, a negotiable note or bill of exchange.
An instrument obligating the payment of money which is transferable from one person to another by endorsement and delivery or by delivery only.
A negotiable instrument is one that can be bought and sold after being issued - in other words, it is a tradeable instrument. Negotiability is a prerequisite of secondary markets.