What is pre-trade transparency?

Pre-trade transparency refers to the publication of data concerning orders sitting in the order book. A market with full pre-trade transparency is a market in which the relevant data – at least, price and size (and eventually date/time of the order entry) – of all orders sitting in the book are publicly available. 

For example, if you or I wish to trade on a major trading venue (such as Euronext or LSE – in which there is a high level of pre-trade transparency), we can see the price and size of the orders sitting in the book and therefore know what will be the outcome of our order entry (i.e., whether it will be immediately fulfilled or added to the sitting orders). (You and I may only be able to see the best-priced orders sitting in the book since we do not pay for premium data services, but the professional investment community will see all orders.)

As an opposite example, block trading venues provide no information regarding the orders sitting in the book, and therefore investors place orders without knowing what will be the outcome of their order entries.