Order book depth

Order book depth measures the quantity of resting limit orders at each price increment on both the bid side (buy orders below the current price) and the ask side (sell orders above it), typically shown as a ladder for a fixed number of levels out from the best bid and ask.

A thick wall of resting size at one price level can act like support or resistance, since a market order would need to fully consume that size before price can trade through it. But depth is genuinely live: size can appear or vanish in fractions of a second as orders get placed and pulled. Watching how a wall behaves as price approaches it, whether it holds and absorbs incoming market orders or gets pulled just before price arrives, tells you more than the wall's static size alone. Depth is also usually shown alongside the bid-ask spread itself, since a wide spread with thin depth on both sides implies a much more fragile market than a tight spread backed by size several levels deep.

Order book depth is a snapshot of resting intent, not a guarantee of what happens when price gets there. A large resting order can be pulled the instant it's tested, a practice sometimes called spoofing, which makes the book look more supportive or resistant than it actually is. Depth also only shows orders sitting on that specific venue's book; it says nothing about resting liquidity elsewhere, or about market orders about to arrive that haven't been placed yet. A thin book can also simply mean low current interest at that moment, not an imminent breakout.

Bitcoin trades at $114,050 with a resting wall of 340 BTC in sell orders sitting at $114,300, noticeably larger than the 20 to 40 BTC typically resting at nearby levels. As price grinds up toward that level, the wall initially holds, absorbing incoming market buys without moving, consistent with genuine resistance. If a fresh wave of aggressive buying manages to fully consume that 340 BTC and price trades through $114,300 anyway, the level that looked like a ceiling is gone, and the next resting cluster further up becomes the level to watch instead.