Liquidity
Liquidity measures how easily you can buy or sell an asset quickly and in size without moving its price. The higher it is, the tighter the spread.
A liquid market has plenty of buyers and sellers at every moment. EUR/USD during the London session is the textbook example: you can place an order for several lots without shifting the price.
When liquidity drops, overnight, on public holidays, at year end or just before a release, spreads widen, slippage increases and moves become more erratic.
The word is also used in price action: “liquidity zones” are the areas where stops pile up, for example just above an obvious high. Price often reaches for them before turning around. Knowing when liquidity is thin helps you decide when not to trade at all.