What is Qualified Institutional Buyer?

Legal Definition
A qualified institutional buyer (QIB), in United States law and finance, is a purchaser of securities that is deemed financially sophisticated and is legally recognized by securities market regulators to need less protection from issuers than most public investors. Typically, the qualifications for this designation are based on an investor's total assets under management and specific legal conditions in the country where the fund is located. Rule 144A requires an institution to manage at least $100 million in securities from issuers not affiliated with the institution to be considered a QIB. If the institution is a bank or savings and loans thrift they must have a net worth of at least $25 million. If the institution is a registered dealer acting for its own account it must in the aggregate own and invest on a discretionary basis at least $10 million of securities of issuers not affiliated with the dealer.

Certain private placements of stocks and bonds are made available only to qualified institutional buyers to limit regulatory restrictions and public filing requirements.
-- Wikipedia
Legal Definition
An investor allowed to trade privately securities not offered on the public market. It adds liquidity by getting rid of unmarketable securities. These sales are allowed by the US as long as the contract agreed upon is met.