Bid-ask spread

The bid-ask spread is the gap between the highest price someone will pay to buy (bid) and the lowest price someone will accept to sell (ask). It's the single best proxy for an asset's liquidity: a narrow spread (AAPL: 1 cent) means deep markets and cheap trading, a wide spread (thin stock: 50 cents) means low volume and expensive round-trips. For a market maker the spread is where profit comes from; for everyone else it's a tax on every trade. Always visible at the top of the order book, and bounded from below by tick size.