Liquidity

Liquidity describes how easily you can buy or sell something without your own order moving the price against you. A liquid market has lots of other participants willing to trade at or near the current price, right now, in size. An illiquid market has few of them, so even a modest order can push the price around.

Mechanically, liquidity lives in the order book: the stack of buy orders (bids) and sell orders (asks) waiting at various prices around the current market price. If there's a thick pile of orders sitting close to the price you want, you can trade a decent amount and get filled near that price. If the book is thin, your order may have to "walk up" or "walk down" through several price levels to get fully filled, which means you pay a worse average price than you expected — this effect is called slippage.

A useful nuance: liquidity is about standing, unfilled orders waiting to trade, while volume is about trades that have already happened. A stock can have huge volume today (lots of shares changed hands) but still have thin liquidity at this exact moment if the order book has emptied out between prints. Conversely, a stock can look quiet on volume but still have a deep, tight book if large resting orders haven't been touched yet. The two are related — high volume names tend to attract more resting orders over time — but they are not the same measurement, and traders sometimes conflate them.

What trips people up is that liquidity is not a single fixed number; it's a live, constantly shifting condition. It can dry up in seconds around news, at the open or close, or when market makers pull their orders, even in a stock that is normally liquid all day. A tight bid-ask spread (the gap between the best buy and best sell price) is a rough sign of good liquidity, but spread alone doesn't tell you how much size is actually available behind that price.