Depth on an order book
The book lists resting orders at each price. Depth at a given distance from the mid price is the cumulative size available within it. A market order consumes that depth from the best price outward, so your average fill worsens as you go.
A tight spread with nothing behind it looks liquid and is not — the first order consumes the whole visible size.
Depth on an AMM
There is no book, so depth is the pool reserves and the shape of the curve. The practical equivalent is price impact: the percentage you lose by trading a given size, which is exactly what depth means in effect.
Concentrated liquidity makes this price-dependent — a pool can be deep at the current price and thin a few percent away.
Measuring it before you trade
Request a quote at your actual size and read the price impact. Then request one at double the size. The way impact scales tells you whether you are near the edge of available liquidity.
This takes seconds and is more informative than any liquidity metric published elsewhere.
Why it varies by venue and chain
Liquidity is fragmented across chains and venues, and it does not aggregate. The same pair can be deep on one network and nearly untradeable on another, and an aggregator can only route to the pools that exist where you are.
FBT Swap shows the price impact for the amount you entered on the network you selected, which is the only depth measurement that matters for your specific trade.