Liquidity
Definition
Liquidity in trading refers to how easily you can enter and exit a position at a fair price — a liquid stock has high volume and a tight bid-ask spread, allowing large orders to be filled quickly without moving the price, while an illiquid stock can move dramatically just from your own order.
Example
“I tried to exit 5,000 shares of a $2 stock and my sell order moved the bid down $0.30 before I was even half filled — that's what illiquidity does to you when you're trying to get out fast.”
Detailed Explanation
Liquidity is one of the most underappreciated concepts for new traders. Everyone focuses on finding the right setup, but getting into and out of a trade is a physical process — and if there aren't enough buyers when you're selling or sellers when you're buying, the market will make you pay a steep price in slippage to get your order done. High-liquidity stocks like SPY, AAPL, or NVDA have millions of shares trading per day with spreads of a penny or less. Low-liquidity stocks might trade 50,000 shares per day with spreads of 5–10 cents or more.
Volume is the primary indicator of liquidity, but it's not the whole story. You also want to look at the average dollar volume (shares × price), the bid-ask spread as a percentage of price, and how deep the order book is at each level. A stock trading 500,000 shares at $0.50 has the same volume as one trading 500,000 shares at $50, but the $50 stock is trading $25 million per day vs. $250,000 — very different liquidity profiles in practice. Relative volume (current volume vs. average volume) tells you whether today is a high-liquidity day or a quiet one even for a normally liquid name.
For position sizing, liquidity creates hard constraints. A common rule is never to trade a position larger than 1–2% of a stock's average daily volume — otherwise your own order impacts price significantly. If a stock does $2 million average daily volume and you're trying to buy $200,000 worth in one shot, you'll likely move the price against yourself during entry. This is why institutional traders use algorithms to slice large orders into tiny pieces over time — they're managing their own market impact. As a retail trader, respecting liquidity limits is how you avoid paying a hidden tax on every trade.