Liquidity and Market Depth

Liquidity is the quality of being able to buy or sell a meaningful size quickly without shifting the price much. Market depth is how you see it: the order book stacks the resting buy orders below the current price and the resting sell orders above it, showing how much size waits at each level. A deep book has large quantities resting close to the market; a thin one has little. Where volume tells you how much has traded, depth tells you how much is available to trade right now.

When your order arrives, it consumes the resting liquidity at the best price, then the next price, and so on until it is filled. In a deep book your order is absorbed at or near one price with little movement. In a thin book the same order eats through several levels, and each level it consumes pushes the price further from where you started. This is why the identical order can fill cleanly on a liquid instrument and slip badly on a thin one: the depth behind the price, not the price itself, decides the cost.

Liquidity is not constant. It is deepest during the most active hours, when many participants are quoting, and it thins overnight, on holidays, and in the seconds around news when market makers pull their orders. A volume profile can hint at the durable side of this, but only as an indirect proxy: it shows where volume has already traded in the past, not the live resting orders in the book. Prices that absorbed heavy volume historically tend to be areas where liquidity congregates and price moves slowly, while thin-volume prices are where it travels fast. The order book shows current depth; the volume profile only suggests where liquidity tends to gather.