Wiktionary
n. (context legal banking business English) A right to receive payment of money which is unconditional (sometimes excepting loss or theft) and capable of transfer by negotiation.
WordNet
n. an unconditional order or promise to pay an amount of money
Wikipedia
A negotiable instrument is a document guaranteeing the payment of a specific amount of money, either on demand, or at a set time, with the payer named on the document. More specifically, it is a document contemplated by or consisting of a contract, which promises the payment of money without condition, which may be paid either on demand or at a future date. The term can have different meanings, depending on what law is being applied and what country it is used in and what context it is used in.
Examples of negotiable instruments include promissory notes, bills of exchange, banknotes, demand draft and cheques.
Because money is promised to be paid, the instrument itself can be used by the holder in due course as a store of value. The instrument may be transferred to a third party; it is the holder of the instrument who will ultimately get paid by the payer on the instrument. Transfers can happen at less than the face value of the instrument and this is known as discounting; e.g., this may happen if there is doubt about the payer's ability to pay.
Usage examples of "negotiable instrument".
It has to be in a bank and then wire transferred to another bank, or be put into a negotiable instrument, like a cashier’.
And this thing is what they call a negotiable instrument, it's like a big check you don't have to sign.