An order that is placed or posted for a sale or purchase of a large number of securities in one transaction. The size of a block order exceeds that of standard size orders as defined by an exchange or a trading venue. A block order involves trading in a large number of securities  at the same time, usually for amounts not less than a specific amount in market value or a specific number of shares, depending on the market through which the transaction will be cleared.

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 order. A block order 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 order 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 orders 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.

It also referred to as a block trade.