A stock transaction that fits one of the following two criteria:

1. A stock trade involving a security that does not trade on a major exchange, i.e., an over-the-counter (OTC) stock.

2. A stock trade involving a stock that is listed on a major exchange but is still executed over the counter, typically between two institutions or an institution and a customer. This type of off-board trade can be done only with 19c3 securities, or stocks that listed on the exchange after 1979.




Taobiz explains Off Board
Off-board trades of the latter type will usually involve two brokerage firms or other institutions and involve a large number of shares. The two parties will decide to do a trade off board to prevent order distortions within the underling stock, as well as keep relative anonymity in the broad markets.

The term “off board” refers to the fact that the venerable New York Stock Exchange is commonly known as the Big Board.