Quick definition Liquidity is the ability to buy or sell a security quickly at or near the current market price without significantly moving the price. A liquid security has tight spreads and deep order books. An illiquid security has wide spreads and thin liquidity. What it is Liquidity has three dimensions: tightness (how wide the spread is), depth (how much volume is available at the best prices), and resilience (how quickly the order book rebuilds after a large trade). A liquid security like Apple might have a spread of 0.01 dollars with 1 million shares available on each side of the order book. An illiquid security might have a spread of 0.50 dollars with only 10,000 shares available. Why it matters Liquidity determines transaction costs. Trading illiquid securities costs more because you must offer wider prices or accept worse fills to move your order size. Traders avoid illiquid securities for this reason. Liquidity also affects volatility. Illiquid securities experience larger price swings because fewer participants are willing to hold inventory. Liquid securities attract more market makers, which stabilises prices. Liquidity across venues Large exchanges (NYSE, NASDAQ) are more liquid than smaller exchanges. Blue-chip stocks (Apple, Microsoft) are more liquid than small-cap stocks. Forex pairs like EUR/USD are highly liquid; exotic currency pairs are less liquid. Time of day also matters; trading volume is highest during core hours (10 AM to 3 PM ET) and lowest at market open and close. Practical example You want to buy 100,000 shares of Tesla (highly liquid, average daily volume 100 million shares). The spread is 0.02 dollars and you can buy 500,000 shares at the ask without moving the price significantly. Your market impact is minimal. You want to buy 100,000 shares of a small-cap stock (illiquid, average daily volume 500,000 shares). The spread is 0.50 dollars and only 50,000 shares are available at the best ask. To buy all 100,000 shares, you must move up the order book and pay prices 1 to 2 dollars higher. Your market impact is large. Liquidity provision Market makers provide liquidity by standing ready to buy and sell at the prices they quote. They profit from the spread but bear the risk of holding inventory. In highly liquid markets, competition among market makers keeps spreads tight. See also - Bid-Ask Spread - Order Book Depth - Market Impact - Market Maker
