A large lot of securities (shares, bonds, etc.) that are traded in a single transaction. Typically, a lot consisting of 1,000 shares at least or a bond transaction with a value of USD 500,000 or more is deemed to be a block trade. A block trade is a high-volume transaction, in a given issue, that is privately negotiated between market participants who enter into it outside of the public auction market (open market).. It may also be consummated via a broker. However, a block trade would not be considered privately negotiated if it was transacted on a system that operated like a trading venue or central limit order book.

Block trades executed directly between counterparties based on electronically displayed indicative markets must be conducted only through direct bilateral communications in which the broker also takes part, where applicable, or the parties involved.

Block trading services (i.e., upstairs trading desks) are often provided to institutional clients by major broker-dealers.