What is Bearer Instrument?

Legal Definition
A bearer instrument is a document that entitles the holder of the document rights of ownership or title to the underlying property, such as shares or bonds. Bearer instruments differ from normal registered instruments, in that no record is kept of who owns the underlying property, or of the transactions involving transfer of ownership. Whoever physically holds the bearer document is assumed to be the owner of the property. This is useful for investors and corporate officers who wish to retain anonymity, but ownership (or legal entitlement) is extremely difficult to establish in event of loss or theft.

In general, the legal situs of the property is where the instrument is located. Bearer instruments can be used in certain jurisdictions to avoid transfer taxes, although taxes may be charged when bearer instruments are issued.

In the United States, under the Uniform Commercial Code, a negotiable instrument (such as a check or promissory note) that is payable to the order of "bearer" or "cash" may be enforced (i.e. redeemed for payment) by the party in possession. The payee (i.e. the person named in the "pay to" line) may also convert an instrument into a bearer instrument by endorsing (signing) the back. In practice, however, many merchants and financial institutions will not pay a check presented for payment by anyone other than the named payee.

Bearer shares are banned in some countries, because of their potential for abuse, such as tax evasion, movement of funds, and money laundering. In 1982, the United States ended federal tax deductions for interest paid on bearer bonds.
-- Wikipedia
Legal Definition
An instrument that can be paid to any holder no matter who they are.