Top of Book
a.k.a. Best bid and offer, BBO, Inside market
The best available buy price and best available sell price in an order book right now.
In plain English
The best available buy price and best available sell price in an order book right now.
Top of book is the front row of the order book: the highest bid and the lowest ask, along with the size available at each. The gap between them is the tightest spread on offer.
It shows only a sliver of the market. A large order may chew through the top of book and fill at worse prices deeper down, which is why big traders care about the full "depth" too.
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