A document of title that can be freely negotiated. It usually concerns choses-in-action. Title can be transferred by delivery, and no particular form is necessary, bills of exchange, cheques, promissory notes, are all ^ negotiable instruments. It should, however, be noted that holders of a negotiable instrument cannot pass on a better title than they themselves possess. This would be relevant in instances of theft.
Reference: The Penguin Business Dictionary, 3rd edt.