Tick-by-tick (TBT) data is the most detailed form of market data. It contains a message for every event in the market: each new order, each change or cancellation of an order, and each trade, stamped with the time at which the exchange processed it. From this stream a recipient can rebuild the complete order book at any instant, including the queue of orders at each price. The label TBT is used in particular by exchanges in India for their order-level feeds; elsewhere the same kind of feed is called market-by-order or full-depth data.
The contrast is with snapshot data, which shows the market only at intervals. Tick-by-tick feeds are essential for high-frequency trading, for studying order flow and for accurate simulation of order execution. They are costly to use: message rates are very high, firms usually receive them through colocation, and historical archives run to enormous sizes.