Liquidity

A market is liquid when large orders execute quickly at prices close to the last trade. Concretely, liquidity lives in the order book: tight spreads, thick resting size near the touch, and fast replenishment after trades consume it.

Why traders care

The same order pays more slippage in a thin market, so liquidity is the denominator of every execution-cost estimate. It also caps a strategy's size: a system that earns 20 bps per trade can only scale to whatever the book absorbs for less than 20 bps, and that ceiling usually arrives long before the signal stops working. And liquidity evaporates in stress, exactly when everyone wants out, which is why fills modeled on normal-market depth understate tail risk.

A second meaning

In price-action vocabulary, "liquidity" also refers to clusters of resting stop orders (below an obvious swing low, say) that large players are said to target before reversing. That usage belongs to market structure analysis rather than microstructure, but you'll meet both in trading writing.

On AlphaProve

Liquidity capping a strategy's size maps onto concrete engine controls. Portfolio kill-switches — max_gross_exposure_pct, max_concentration_pct, and max_open_positions — bound how much you can hold at once, while the sizing modes set each trade's size, together keeping a system inside what a real book could absorb. Where reconstructed L2 depth exists the engine prices fills against it too, so an order that outruns the resting size visibly pays for the thin book instead of filling at a clean candle price.