Market depth is how much size rests near the current price in an order book or liquidity pool. Deep markets absorb larger buys and sells with less price impact. Thin markets move when modest flow arrives. Depth is not the same as last price or 24-hour volume. Volume can look busy while resting size near the mid is empty.
Why it matters
Narratives travel faster than exits. A token can trend on social feeds while the book a few percent away is hollow. That is when market orders become the news. Traders who only watch candles miss whether they can enter and leave near the quote they see.
Depth also changes by venue and by time of day. A major pair on a large exchange can look thick. The same asset on a smaller venue, or during a holiday session, can thin out. Alts usually have less depth than bitcoin and ether. That is why alt liquidity stories matter even when bitcoin looks calm.
Screenshot walls can lie. Spoofed size may cancel before it trades. Useful depth is size that actually fills and replenishes. In automated market makers, depth shows up as pool size and curve shape rather than a classic book, but the question is the same: how far does price move for your clip.
Depth metrics should be sized to your clip. A book that is deep for a five-thousand-dollar order can be thin for a five-hundred-thousand-dollar exit. Measure impact in percent terms for the size you actually trade, across the venues you can access, at the hours you trade. If you need multiple venues to complete an exit, your true depth is fragmented and your operational risk rises with every hop.
Also separate displayed depth from actionable depth after fees and latency. A fill that looks fine in a static book can worsen once your order is visible and other participants cancel or race you.
Example
Imagine the mid price is $100. If $2 million of bids sit within 1% below and $2 million of offers within 1% above, a $50,000 market buy barely moves the tape. If only $20,000 sits in that band, the same order walks the book and prints a worse average. Slippage is the bill you pay for thin depth. Related: liquidity, slippage, order book, and market structure.