An 'off-book' trade is one in which shares are traded outside of an exchange or regulated body. They are typically carried out through the over-the-counter (OTC) market. Off-book transactions are made between two parties directly, outside of the order books.
After the two parties have agreed on a price, it is customary for one of the participants to report the trade and its parameters to the exchange - and wider market - in order to bring the execution 'on exchange.'
When you trade on an exchange, the transaction is recorded directly on the order book, increasing visibility for other market participants.

In financial markets, “off-book” describes trading activity that occurs outside the main order book used by an exchange. Rather than having a buy and sell order matched electronically through the public market, participants can negotiate the terms directly or arrange execution with the assistance of a broker. This can provide greater flexibility for certain types of transactions.

Off-book trading is often associated with larger orders because executing a substantial position openly may affect the market price. By arranging the transaction separately, participants may be able to reduce immediate market impact. Nevertheless, the completed trade may still need to be reported under applicable exchange or regulatory rules.

The significance of off-book transactions varies between markets. Some markets have specific procedures for reporting or identifying these trades, while others use different terminology. Understanding how off-book activity works can help investors interpret trading information and recognize that not every market transaction will necessarily appear in the central order book.