Liquidity
How easily an asset can be bought and sold quickly without moving the price much. High liquidity = tight spread, low slippage.
Major pairs, large index futures and BTC are the most liquid instruments. Liquidity varies by session: EURUSD is less liquid in the Asian session than in the London session.
In the order flow approach, "liquidity" also refers to clusters of resting orders (stop clusters, equal highs and lows); price moving toward these zones is interpreted as a "liquidity sweep."